Question 1: A 30-year-old woman develops a high fever, diffuse erythematous rash, and hypotension during menses. Blood cultures are negative. Which mechanism best explains systemic toxicity in this condition?

• Type III hypersensitivity immune complex deposition
• Superantigen cross-linking MHC II and T-cell receptor (Correct answer)
• Lipopolysaccharide binding TLR-4
• Exotoxin blocking acetylcholine release

Correct answer: Superantigen cross-linking MHC II and T-cell receptor

TSST-1 is a superantigen that cross-links MHC class II on APCs with the Vβ region of T-cell receptors, causing massive non-specific T-cell activation and cytokine storm.

Question 2: A 28-year-old man with a history of intravenous drug use comes to the physician because of a 6-week history of fever, nonproductive cough, chills, and progressive shortness of breath. His temperature is 39°C (102.2°F), pulse is 110/min, respirations are 32/min and regular, and blood pressure is 120/80 mm Hg. Physical examination shows a white, patchy, loosely adherent exudate on the buccal mucosa bilaterally. A chest x-ray shows bilateral interstitial infiltrates. After receiving treatment for pneumonia, he agrees to participate in a clinical study of the effects of interleukin-2 (IL-2). After administration of IL-2, which of the following hematologic changes is most likely in this patient?

• Decreased CD4+ T lymphocytes
• Increased CD4+ T lymphocytes (Correct answer)
• Decreased erythrocytes
• Decreased platelet count
• Increased erythrocytes
• Increased platelet count

Correct answer: Increased CD4+ T lymphocytes

The patient's history of HIV and opportunistic infections (Pneumocystis pneumonia, oral candidiasis) indicates significant immunosuppression, typically associated with low CD4+ T lymphocyte counts. Interleukin-2 (IL-2) is a cytokine that stimulates the proliferation, differentiation, and activation of T lymphocytes, particularly CD4+ and CD8+ T cells. Therefore, administration of IL-2 is most likely to lead to an increase in CD4+ T lymphocytes, aiming to bolster the immune response.

Question 3: A 3-year-old girl is brought to the emergency department 2 days after the onset of fever, profuse watery diarrhea, and progressive lethargy. On arrival, she appears ill and is poorly responsive. Her temperature is 39.4°C (103°F), pulse is 180/min, respirations are 40/min, and blood pressure is 80/45 mm Hg. Examination shows poor skin turgor and cool extremities. Mucous membranes are dry. Capillary refill time is delayed. Which of the following is the most appropriate next step to decrease the risk of acute renal failure in this patient?

• Diuretic therapy
• Monitoring urine output
• Vasopressor therapy
• Fluid resuscitation (Correct answer)
• Ceftriaxone therapy

Correct answer: Fluid resuscitation

This 3-year-old girl is in hypovolemic shock due to severe dehydration from profuse watery diarrhea, evidenced by lethargy, hypotension, tachycardia, poor skin turgor, cool extremities, and delayed capillary refill. Aggressive intravenous fluid resuscitation is the most critical immediate step to restore circulating volume, improve perfusion to vital organs, and prevent acute renal failure, which is a common complication of severe dehydration.

Question 4: A patient receiving a β2-agonist inhaler for asthma notices that the same dose produces less bronchodilation after 2 weeks of continuous use. This phenomenon is best described as:

• Tachyphylaxis caused by increased drug metabolism
• Tolerance due to receptor desensitization and downregulation (Correct answer)
• Drug resistance from mutation of the β2 receptor gene
• Idiosyncratic reaction unique to this patient's genotype

Correct answer: Tolerance due to receptor desensitization and downregulation

Prolonged agonist exposure causes receptor phosphorylation, uncoupling from G-proteins, and downregulation of receptor number, reducing drug response over time.

Question 5: Two hours after vaginal delivery at term of a 3062-g (6-lb 12-oz) newborn, a 32-year-old woman, gravida 3, para 3, has the onset of heavy vaginal bleeding. Labor was augmented with oxytocin because of a prolonged first stage and required forceps delivery over a midline second-degree episiotomy. The abdomen is soft and nontender. Examination shows a boggy uterus palpated 4 cm above the umbilicus. The perineum is intact. Which of the following is the most likely cause of this patient’s hemorrhage?

• Uterine infection
• Uterine atony (Correct answer)
• Episiotomy site bleeding
• Uterine rupture
• Disseminated intravascular coagulation

Correct answer: Uterine atony

The most common cause of postpartum hemorrhage is uterine atony, which is characterized by a boggy, enlarged uterus that fails to contract after delivery. Risk factors present in this patient include prolonged labor and oxytocin augmentation. The intact perineum rules out significant lacerations, and the boggy uterus strongly points to atony as the cause of the heavy bleeding.

Question 6: A postpartum patient on day 3 has a temperature of 38.6°C, uterine tenderness on palpation, and foul-smelling lochia. What is the most appropriate antibiotic regimen?

• IV clindamycin plus gentamicin (Correct answer)
• IV ampicillin alone
• Oral metronidazole alone for 10 days
• Oral amoxicillin-clavulanate for 7 days

Correct answer: IV clindamycin plus gentamicin

Postpartum endometritis is treated with IV clindamycin plus gentamicin, which provides coverage for gram-negative and anaerobic organisms.

Question 7: A 16-year-old girl is brought to the emergency department 6 hours after the onset of moderate lower abdominal cramps and intermittent nausea. She has not vomited during this time. She says that her last menstrual period was 2 months ago, but she has had intermittent bleeding since then, including spotting for the past 2 days. Menarche was at the age of 15 years. Menses occur at irregular 25- to 45-day intervals. She is sexually active and uses condoms inconsistently. Her temperature is 38.1°C (100.6°F), pulse is 94/min, respirations are 22/min, and blood pressure is 120/80 mm Hg. Examination shows a soft abdomen with lower quadrant tenderness, especially on the right. Bowel sounds are normal. Pelvic examination shows scant vaginal bleeding and a palpable, tender right adnexal mass. The cervix appears normal. There is no cervical motion tenderness. Which of the following is the most appropriate next step in management?

• Abdominal x-ray
• Ceftriaxone and azithromycin therapy
• Measurement of serum β-hCG concentration (Correct answer)
• Complete blood count
• Exploratory laparoscopy

Correct answer: Measurement of serum β-hCG concentration

Given the patient's amenorrhea, irregular bleeding, sexual activity with inconsistent condom use, lower abdominal pain, and a palpable, tender adnexal mass, an ectopic pregnancy must be ruled out immediately. Measuring serum β-hCG concentration is the most appropriate initial diagnostic step to confirm pregnancy and then guide further evaluation (e.g., ultrasound) to determine its location.

Question 8: A 72-year-old nursing home resident presents with fever, productive cough, and right lower lobe consolidation on CXR. He has dysphagia. Which organism is most likely responsible?

• Streptococcus pneumoniae
• Legionella pneumophila
• Pseudomonas aeruginosa
• Aspiration of oral flora (polymicrobial) (Correct answer)

Correct answer: Aspiration of oral flora (polymicrobial)

Aspiration pneumonia in a dysphagic nursing home patient is typically polymicrobial, involving oral anaerobes and gram-negative organisms.

Question 9: A 21-year-old woman comes to the physician for counseling prior to conception. She delivered a female newborn with anencephaly 1 year ago. The newborn died at the age of 4 days. She asks the physician if she can take any vitamins to decrease her risk for conceiving a fetus with anencephaly. It is most appropriate for the physician to recommend which of the following vitamins?

• Vitamin B2 (riboflavin)
• Folic acid (Correct answer)
• Vitamin B12 (cyanocobalamin)
• Vitamin B6 (pyridoxine)
• Vitamin B1 (thiamine)
• Biotin

Correct answer: Folic acid

Anencephaly is a severe neural tube defect (NTD) that occurs during early fetal development. Supplementation with folic acid (vitamin B9) before conception and during the first trimester of pregnancy has been extensively proven to significantly reduce the risk of NTDs. Folic acid is crucial for DNA synthesis and repair, and for cell division, which are vital processes during neural tube closure.

Question 10: A 3-year-old boy is brought to the physician because of a 2-day history of fever and an itchy rash. The rash began on his face and then spread to his trunk, arms, and legs. Several children at his day-care center have had similar symptoms. His temperature is 38.9°C (102°F), pulse is 100/min, respirations are 20/min, and blood pressure is 110/60 mm Hg. Physical examination shows multiple red papules and vesicles over the face, trunk, and upper and lower extremities. Some vesicles contain clear fluid, whereas others are crusted. Which of the following is the most likely causal virus?

• Human papillomavirus
• Herpes simplex virus 2
• Epstein-Barr virus
• Varicella-zoster virus (Correct answer)
• Cytomegalovirus

Correct answer: Varicella-zoster virus

The clinical presentation of a febrile illness with a highly pruritic rash that starts on the face/trunk and spreads centrifugally, characterized by lesions in various stages of development (papules, vesicles, crusts—'dewdrop on a rose petal' appearance), is classic for varicella (chickenpox). This highly contagious disease is caused by the varicella-zoster virus (VZV) and commonly spreads in settings like day-care centers.

Question 11: A 44-year-old man presents with a 5-cm retroperitoneal mass found incidentally on CT. It is well-circumscribed with no adjacent organ involvement. Core biopsy reveals a well-differentiated liposarcoma. What is the mainstay of treatment?

• Systemic ifosfamide-based chemotherapy
• Observation and repeat imaging in 3 months
• Wide surgical resection with negative margins (Correct answer)
• Radiation therapy alone

Correct answer: Wide surgical resection with negative margins

Wide surgical excision with negative margins is the cornerstone of treatment for retroperitoneal liposarcoma, as it offers the best chance of cure.

Question 12: A 3-year-old girl is brought to the emergency department 30 minutes after she tripped and fell. Physical examination shows blue sclera and edema and tenderness over the right proximal lower extremity. X-rays show a fracture of the right femur, as well as several fractures of varying ages of the left clavicle, right humerus, and right fibula. Which of the following is the most likely diagnosis?

• Marfan syndrome
• Hurler syndrome
• Osteogenesis imperfecta (Correct answer)
• Ehlers-Danlos syndrome
• Achondroplasia

Correct answer: Osteogenesis imperfecta

The combination of blue sclera and recurrent fractures from minimal trauma, especially fractures of varying ages, is pathognomonic for osteogenesis imperfecta (OI). This genetic disorder is caused by defects in type I collagen synthesis, leading to brittle bones that fracture easily. Blue sclera occurs because the thin sclera allows the underlying choroidal veins to show through.

Question 13: Atropine is administered to reverse the effects of organophosphate poisoning. Which toxic effect of organophosphates does atropine NOT reverse?

• Bradycardia
• Bronchospasm
• Excessive secretions
• Skeletal muscle paralysis (Correct answer)

Correct answer: Skeletal muscle paralysis

Atropine blocks muscarinic receptors but has no effect on nicotinic receptors at the neuromuscular junction; pralidoxime (2-PAM) must be used to reverse skeletal muscle effects.

Question 14: A 2-month-old infant has a 5-cm strawberry hemangioma on the cheek that is increasing in size. No other lesions are noted. Which of the following is the most appropriate next step in management?

• Solid carbon dioxide application to the lesion
• Laser therapy to the lesion
• Observation of the lesion (Correct answer)
• Surgical removal of the lesion
• Intralesional corticosteroid treatment

Correct answer: Observation of the lesion

Infantile hemangiomas, commonly known as strawberry hemangiomas, are benign vascular tumors that typically proliferate during the first year of life and then spontaneously involute over several years. For an uncomplicated lesion like this one on the cheek, which is not obstructing vision, breathing, or causing ulceration, observation is the most appropriate initial management, as most resolve without intervention.

Question 15: A 14-year-old girl is brought to the physician because of a recent growth spurt of 15 cm (6 in) during the past year. She also has had increasing fatigue and palpitations during this period. Her paternal aunt has a history of palpitations and severe myopia. She is at the 95th percentile for height and 50th percentile for weight. Physical examination shows a long, thin face. Ophthalmologic examination shows dislocated lenses. Cardiac examination shows a hyperdynamic precordium with early click and systolic murmur. Echocardiography shows an enlarged aortic root and mitral valve prolapse. Abnormal synthesis of which of the following proteins is the most likely cause of this patient’s disorder?

• PAX 6
• Collagen, type I
• Laminin
• Elastin
• Fibroblast growth factor R3
• Fibrillin-1 (Correct answer)
• Neurofibromin

Correct answer: Fibrillin-1

The patient's tall stature, long thin face, dislocated lenses, and cardiovascular abnormalities like an enlarged aortic root and mitral valve prolapse are characteristic features of Marfan syndrome. This autosomal dominant disorder results from a mutation in the *FBN1* gene, which encodes fibrillin-1. Fibrillin-1 is a key component of elastic fibers, and its abnormal synthesis leads to weakened connective tissue throughout the body, particularly affecting the skeletal, ocular, and cardiovascular systems.

Question 16: Antidiuretic hormone (ADH/vasopressin) increases water reabsorption by:

• Inhibiting Na+ reabsorption in the distal tubule
• Increasing GFR
• Inserting aquaporin-2 channels in the collecting duct apical membrane (Correct answer)
• Inserting aquaporin-1 channels in the proximal tubule

Correct answer: Inserting aquaporin-2 channels in the collecting duct apical membrane

ADH stimulates V2 receptors on collecting duct principal cells, triggering cAMP-mediated insertion of aquaporin-2 channels into the apical membrane.

Question 17: A malnourished 45-year-old alcoholic presents with confusion, ophthalmoplegia, and ataxia. What is the most urgent intervention?

• Naloxone IV
• IV glucose bolus
• Lorazepam IV
• IV thiamine before glucose (Correct answer)

Correct answer: IV thiamine before glucose

Wernicke's encephalopathy (thiamine deficiency) requires IV thiamine before glucose; giving glucose first can precipitate or worsen the encephalopathy by depleting remaining thiamine.

Question 18: A gram-positive rod forming spores in soil causes a wound infection with gas formation and tissue crepitus. Which organism is responsible?

• Listeria monocytogenes
• Clostridium perfringens (Correct answer)
• Actinomyces israelii
• Bacillus anthracis

Correct answer: Clostridium perfringens

Clostridium perfringens produces alpha toxin (lecithinase) causing gas gangrene, with gas formation from fermentation in anaerobic tissue.

Question 19: A 22-year-old primigravid woman at 34 weeks’ gestation is brought to the emergency department by ambulance after being found unconscious by her husband. Paramedics report that she was having tonic-clonic movements that have now stopped. Her last visit to the physician was 2 weeks ago. Pregnancy had been uncomplicated. On arrival, she is awake, lethargic, and mildly confused. She says that she had a headache and did not feel well earlier in the day. She has no history of serious illness. Her only medication is a prenatal vitamin. Her temperature is 37°C (98.6°F), pulse is 80/min, respirations are 18/min, and blood pressure is 170/110 mm Hg. Examination shows a nontender, soft uterus consistent in size with a 34-week gestation. Cranial nerves are intact. Motor function is normal. Deep tendon reflexes are 3+. Which of the following is the most likely diagnosis?

• Eclampsia (Correct answer)
• Migraine
• Cerebral infarction
• Absence seizures
• Pheochromocytoma

Correct answer: Eclampsia

The patient's presentation of new-onset tonic-clonic seizures at 34 weeks' gestation, accompanied by severe hypertension (170/110 mm Hg), headache, lethargy, confusion, and hyperreflexia, is classic for eclampsia. Eclampsia is defined as the occurrence of grand mal seizures in a woman with preeclampsia, a condition characterized by hypertension and often proteinuria during pregnancy. The combination of these symptoms in a pregnant woman makes eclampsia the most likely diagnosis.

Question 20: A 25-year-old woman presents with fatigue and jaundice. CBC shows Hgb 8.5, reticulocyte count 9%, and spherocytes on peripheral smear. Direct Coombs test is positive. What is the most likely diagnosis?

• Autoimmune hemolytic anemia (Correct answer)
• Microangiopathic hemolytic anemia
• G6PD deficiency
• Hereditary spherocytosis

Correct answer: Autoimmune hemolytic anemia

A positive direct Coombs test confirms autoantibodies coating the red cells, consistent with autoimmune hemolytic anemia.

Question 21: A 27-year-old nulligravid woman and her husband have been unable to conceive for 12 months. She has never used contraception. Menses occur at 28-day intervals, and her last menstrual period was 2 weeks ago. She had a single episode of pelvic inflammatory disease 4 years ago and was treated with oral antibiotics. Vaginal examination shows no abnormalities. Cervical cultures are normal. Which of the following is the most appropriate next step in diagnosis?

• Sperm penetration assay
• Endometrial biopsy
• Reevaluation in 6 months
• Ultrasonography of the abdomen
• Hysterosalpingography (Correct answer)

Correct answer: Hysterosalpingography

Infertility is defined as the inability to conceive after 12 months of unprotected intercourse for women under 35. Given her history of pelvic inflammatory disease (PID), there is a significant risk of tubal damage and obstruction. Hysterosalpingography (HSG) is the most appropriate next step to evaluate tubal patency and uterine cavity abnormalities, which are common causes of infertility after PID.

Question 22: A 55-year-old man has massive hematemesis. Endoscopy shows a bleeding vessel in a gastric ulcer in the posterior wall of the duodenum. Which artery is most likely eroded?

• Left gastric artery
• Splenic artery
• Superior mesenteric artery
• Gastroduodenal artery (Correct answer)

Correct answer: Gastroduodenal artery

Posterior duodenal ulcers can erode the gastroduodenal artery, which runs behind the first part of the duodenum, causing life-threatening hemorrhage.

Question 23: Three weeks after traveling to California to study desert flowers, a 33-year-old man develops fever, chest pain, and muscle soreness. Two days later, red, tender nodules appear on the shins, and the right ankle is tender and painful. An x-ray of the chest shows a left pleural effusion. Which of the following is the most likely diagnosis?

• Mycobacterium marinum infection
• Mycoplasma pneumoniae infection
• Blastomycosis
• Histoplasmosis
• Coccidioidomycosis (Correct answer)

Correct answer: Coccidioidomycosis

The patient's travel history to California, an endemic region for Coccidioides immitis, combined with symptoms of fever, chest pain, muscle soreness, and the characteristic skin lesions of erythema nodosum (red, tender nodules on the shins), strongly points to coccidioidomycosis. This fungal infection, also known as 'desert fever,' often presents with respiratory symptoms and extrapulmonary manifestations like arthralgias and erythema nodosum, consistent with the patient's findings.

Question 24: A 3629-g (8-lb) male newborn is delivered at term. During the initial examination, urine is found to be leaking from the umbilicus. This patient most likely has an abnormality of which of the following fetal structures?

• Urorectal septum
• Urogenital sinus
• Umbilical arteries
• Urachus (Correct answer)
• Umbilical vein

Correct answer: Urachus

The urachus is an embryonic remnant connecting the fetal bladder to the umbilicus. Normally, it obliterates after birth to become the median umbilical ligament. If it remains patent, it can allow urine to leak from the bladder through the umbilicus, a condition known as a patent urachus.

Question 25: A 17-year-old girl is brought to the physician by her parents 30 minutes after having a generalized tonic-clonic seizure while playing in a soccer game. She currently takes no medications. Physical examination shows no abnormalities. After further testing including 24-hour continuous EEG monitoring, carbamazepine is prescribed. This patient’s use of additional medications should be monitored because of which of the following changes in drug disposition after starting pharmacotherapy?

• Decreased distribution to the brain
• Increased recirculation in the bile
• Decreased absorption in the intestine
• Increased excretion by the kidneys
• Increased metabolism by the liver (Correct answer)

Correct answer: Increased metabolism by the liver

Carbamazepine is a potent inducer of hepatic cytochrome P450 enzymes, particularly CYP3A4. This enzyme induction leads to increased metabolism of many co-administered drugs, as well as carbamazepine itself. Consequently, the plasma concentrations of other medications taken by the patient may decrease, potentially reducing their efficacy and requiring dose adjustments.

Question 26: A 38-year-old woman has hypertension, hypokalemia, and suppressed plasma renin. CT shows a 1.5 cm left adrenal adenoma. What does the tumor cell secrete in excess?

• Epinephrine
• Cortisol
• Aldosterone (Correct answer)
• Androstenedione

Correct answer: Aldosterone

Primary hyperaldosteronism (Conn syndrome) due to adrenocortical adenoma causes excess aldosterone secretion, leading to sodium retention, hypertension, and hypokalemia with suppressed renin.

Question 27: A patient sustains a left hemisection of the thoracic spinal cord (Brown-Séquard). Which finding is expected below the lesion?

• Bilateral loss of all sensory modalities
• Right-sided spastic paralysis with left-sided proprioceptive loss
• Left-sided loss of pain/temperature; right-sided loss of proprioception
• Left-sided loss of proprioception; right-sided loss of pain/temperature (Correct answer)

Correct answer: Left-sided loss of proprioception; right-sided loss of pain/temperature

Brown-Séquard: ipsilateral (left) UMN weakness and dorsal column loss because those tracts are uncrossed at that level; contralateral (right) spinothalamic (pain/temp) loss because the STT crosses within 1-2 spinal levels.

Question 28: The endothelium lining blood vessels is classified as which epithelial type?

• Stratified squamous
• Simple cuboidal
• Simple columnar
• Simple squamous (Correct answer)

Correct answer: Simple squamous

Vascular endothelium is simple squamous epithelium (mesothelial origin), forming a smooth, low-friction lining that minimizes turbulence.

Question 29: A 40-year-old woman presents with a 3-month history of believing her husband is being unfaithful despite no evidence. She is otherwise well-functioning. No hallucinations are present. Which diagnosis is most appropriate?

• Delusional disorder, jealous type (Correct answer)
• Paranoid schizophrenia
• Shared delusional disorder
• Paranoid personality disorder

Correct answer: Delusional disorder, jealous type

Delusional disorder involves a non-bizarre delusion lasting at least 1 month with preserved functioning and no other psychotic symptoms.

Question 30: A 62-year-old man comes to the physician because of burning pain and tenderness of his right great toe 1 day after heavy ethanol consumption. Physical examination shows erythema, swelling, warmth, and tenderness of the right great toe. After a 2-week course of nonsteroidal anti-inflammatory drug treatment, his symptoms decrease in severity but do not completely resolve. The serum concentration of which of the following is most likely increased in this patient?

• Uric acid (Correct answer)
• Phosphate
• Calcium
• Iron
• Creatinine
• Orotic acid
• Carotene

Correct answer: Uric acid

The patient's symptoms of acute, severe pain, swelling, warmth, and erythema in the great toe, particularly after heavy ethanol consumption, are classic for an acute attack of gout. Gout is caused by the deposition of monosodium urate crystals in joints, which occurs when serum uric acid concentrations are elevated. Ethanol consumption can exacerbate gout by increasing uric acid production and decreasing its renal excretion.

Question 31: Ten months after starting procainamide therapy for cardiac arrhythmias, a 56-year-old man develops arthritis and other symptoms consistent with drug-induced systemic lupus erythematosus. Results of a blood test are positive for antinuclear antibodies. This finding is consistent with which of the following genetic polymorphisms in drug metabolism?

• Debrisoquine 4-hydroxylase deficiency
• Glucose 6-phosphate dehydrogenase deficiency
• Phenytoin hydroxylase deficiency
• Erythrocyte catechol-O-methyltransferase deficiency
• Slow acetylator phenotype (Correct answer)

Correct answer: Slow acetylator phenotype

Procainamide-induced systemic lupus erythematosus is strongly associated with the slow acetylator phenotype. Individuals with this genetic polymorphism have reduced activity of N-acetyltransferase, an enzyme responsible for metabolizing procainamide. This leads to higher and prolonged plasma concentrations of the drug and its active metabolites, increasing the risk of developing the autoimmune reaction.

Question 32: Which enzyme catalyzes the rate-limiting step of the pentose phosphate pathway and is important for NADPH production in red blood cells?

• Glucose-6-phosphate dehydrogenase (Correct answer)
• 6-Phosphogluconate dehydrogenase
• Transketolase
• Ribose-5-phosphate isomerase

Correct answer: Glucose-6-phosphate dehydrogenase

G6PD is the rate-limiting enzyme of the hexose monophosphate shunt, producing NADPH essential for glutathione reduction in RBCs.

Question 33: A 32-year-old heroin user presents to the ED in acute opioid withdrawal with severe anxiety, mydriasis, piloerection, and cramping. Which agent reduces autonomic symptoms of opioid withdrawal?

• Methadone
• Buprenorphine
• Naloxone
• Clonidine (Correct answer)

Correct answer: Clonidine

Clonidine, an alpha-2 agonist, suppresses the noradrenergic storm driving autonomic symptoms of opioid withdrawal.

Question 34: A premature infant born at 28 weeks develops increasing respiratory distress immediately after birth. CXR shows diffuse ground-glass opacities bilaterally. The deficiency responsible is:

• Alpha-1 antitrypsin
• Pulmonary IgA
• Clara cell secretory protein
• Surfactant (Correct answer)

Correct answer: Surfactant

Respiratory distress syndrome in premature infants results from surfactant deficiency, which reduces alveolar surface tension.

Question 35: Ligand binding to a Gs-protein coupled receptor primarily leads to:

• Activation of adenylyl cyclase and increased intracellular cAMP (Correct answer)
• Direct DNA binding to regulate transcription
• Direct opening of ligand-gated ion channels
• Activation of phospholipase C and IP3/DAG production

Correct answer: Activation of adenylyl cyclase and increased intracellular cAMP

Gs-coupled receptors activate adenylyl cyclase via the Gαs subunit, increasing cAMP and activating protein kinase A.

Question 36: Which capillary type, found in the kidney glomerulus and small intestine, has pores in its endothelium that enhance permeability?

• Continuous capillaries
• Sinusoidal capillaries
• Lymphatic capillaries
• Fenestrated capillaries (Correct answer)

Correct answer: Fenestrated capillaries

Fenestrated capillaries have transcellular pores (fenestrae) that allow rapid passage of water, ions, and small molecules, critical for filtration and absorption.

Question 37: A 40-year-old woman with systemic lupus erythematosus has a kidney biopsy showing 'wire loop' lesions. What is deposited in these capillary walls?

• Amyloid fibrils
• Anti-GBM antibodies
• IgG and C3 immune complexes (Correct answer)
• IgA deposits

Correct answer: IgG and C3 immune complexes

Wire loop lesions in lupus nephritis (class IV) represent subendothelial immune complex deposits causing marked capillary wall thickening.

Question 38: A 7-year-old with medulloblastoma undergoes posterior fossa resection. Postoperatively, she develops cerebellar mutism, ataxia, and emotional lability. The likely cause is injury to the:

• Hippocampus
• Reticular activating system
• Dentate nucleus and efferent pathways (Correct answer)
• Cerebral cortex

Correct answer: Dentate nucleus and efferent pathways

Posterior fossa syndrome/cerebellar mutism results from injury to the dentate nucleus or its efferent pathways during medulloblastoma resection.

Question 39: A 35-year-old woman is admitted to the hospital because of fever and dry cough for 3 days. An x-ray of the chest shows abnormal findings. Laboratory tests show cold agglutinating antibodies in serum. Results of routine cultures of blood and sputum are negative. Which of the following is the most likely pathogen?

• Streptococcus pneumoniae
• Mycoplasma pneumoniae (Correct answer)
• Mycobacterium tuberculosis
• Staphylococcus aureus
• Influenza virus

Correct answer: Mycoplasma pneumoniae

The patient's presentation of a dry cough, fever, abnormal chest x-ray, and especially the presence of cold agglutinating antibodies in serum, strongly suggests Mycoplasma pneumoniae infection. Mycoplasma pneumoniae is a common cause of 'walking pneumonia,' characterized by a persistent dry cough and often occurring in younger adults. Routine bacterial cultures are typically negative because Mycoplasma lacks a cell wall and requires special media for growth, and cold agglutinins are a common, though non-specific, finding in these infections.

Question 40: A 29-year-old man is brought to the emergency department because he has a severe bilateral headache and irritability. His pulse is 120/min, respirations are 30/min, and blood pressure is 200/120 mm Hg. Ophthalmoscopic examination shows blurring of the optic discs. Deep tendon reflexes are 3+ and symmetric. Which of the following is the most likely diagnosis?

• Alcohol intoxication
• Heavy metal poisoning
• Hyperthyroidism
• Hypertensive encephalopathy (Correct answer)
• Panic disorder

Correct answer: Hypertensive encephalopathy

The acute onset of severe headache, irritability, and neurological symptoms (blurring of optic discs, indicating papilledema, and hyperreflexia) in the setting of extremely elevated blood pressure (200/120 mm Hg) is characteristic of hypertensive encephalopathy. This condition is a medical emergency caused by a rapid and severe increase in blood pressure that overwhelms the brain's autoregulatory capacity, leading to cerebral edema and neurological dysfunction.

Question 41: A demonstration is performed during a lecture on muscle physiology in which a student is asked to fully extend his right arm with the palm up. Two large textbooks are placed on his palm, one at a time. Which of the following facilitates the maximum amount of tension that allows the student to keep his arm extended in place under the increasing weight of the books?

• Amplitude of the action potential
• Amount of Ca 2+ released from the sarcoplasmic reticulum
• Number of motor units recruited (Correct answer)
• Rate of cross-bridge recycling
• Amount of muscle phosphocreatine

Correct answer: Number of motor units recruited

To generate increasing amounts of force or tension in a muscle, the nervous system employs a mechanism called motor unit recruitment. As more weight is added, the brain activates a greater number of motor neurons, which in turn activate more motor units and thus more muscle fibers. This allows for a graded increase in muscle tension, enabling the student to counteract the increasing load and keep his arm extended.

Question 42: A 23-year-old woman started sulfamethoxazole-trimethoprim 10 days ago. She now presents with widespread erythematous targetoid lesions and mucosal ulceration involving her eyes, mouth, and genitalia. Nikolsky sign is positive. What is the most likely diagnosis?

• Erythema multiforme major
• Stevens-Johnson syndrome / Toxic epidermal necrolysis (Correct answer)
• Staphylococcal scalded skin syndrome
• Pemphigus vulgaris

Correct answer: Stevens-Johnson syndrome / Toxic epidermal necrolysis

Stevens-Johnson syndrome/TEN is a severe mucocutaneous drug reaction characterized by extensive epidermal detachment, mucosal involvement, and positive Nikolsky sign.

Question 43: A 57-year-old woman comes to the physician because of a 2-year history of difficulty sleeping. After she gets into bed at night, her legs feel cold and crampy, and she cannot settle into a comfortable position. Walking around temporarily relieves her symptoms. She also has difficulty sitting for a prolonged period of time. She has a mild anxiety disorder but takes no medications. Vital signs are within normal limits. Examination shows no abnormalities. Which of the following is the most likely diagnosis?

• Stiff-person syndrome
• Restless legs syndrome (Correct answer)
• REM sleep behavior disorder
• Major depressive disorder
• Parkinson disease
• Benign fasciculations

Correct answer: Restless legs syndrome

The patient's description of uncomfortable sensations in her legs, particularly at night, that compel her to move them, and which are temporarily relieved by walking, is the hallmark of restless legs syndrome (RLS). RLS is a neurological disorder characterized by an irresistible urge to move the legs, often accompanied by unpleasant sensations, typically worse during periods of rest and in the evening or night.

Question 44: A previously healthy 4-year-old boy is brought to the emergency department 8 hours after the onset of fever and a diffuse, constant headache. He had been well until 2 days ago when he developed malaise. Yesterday, he slept more than usual and had temperatures to 39.2°C (102.5°F). Immunizations are up-to-date. He is lethargic and irritable when aroused. He holds his head extended and cries when his neck is flexed. Muscle strength is normal, and deep tendon reflexes are symmetrically increased. A lumbar puncture yields turbid cerebrospinal fluid. Laboratory studies show: Serum glucose 120 mg/dL Cerebrospinal fluid Opening pressure 250 mm H2O Glucose 10 mg/dL Protein 85 mg/dL Leukocyte count 750/mm3 Segmented neutrophils 95% Lymphocytes 5% Erythrocyte count 0/mm3 A Gram stain of cerebrospinal fluid shows gram-negative diplococci. A CT scan of the head shows no abnormalities. Which of the following is the most likely causal organism?

• Varicella-zoster virus
• Cytomegalovirus
• Haemophilus influenzae
• Streptococcus agalactiae (group B)
• Neisseria meningitidis (Correct answer)

Correct answer: Neisseria meningitidis

The patient's clinical presentation (fever, headache, lethargy, nuchal rigidity) combined with the cerebrospinal fluid (CSF) findings (turbid, high opening pressure, low glucose, high protein, high leukocyte count with neutrophil predominance) is characteristic of bacterial meningitis. The Gram stain showing gram-negative diplococci specifically identifies *Neisseria meningitidis* as the most likely causal organism in this age group.

Question 45: A 70-year-old man is brought to the emergency department because of a 1-week history of increasingly severe left-sided lower abdominal pain and passing gas in his urine. His temperature is 38.9°C (102°F). A CT scan of the abdomen shows gas in the bladder and edema of the fat surrounding the sigmoid colon. There is no obvious lymph node enlargement. Which of the following is the most likely cause of this patient’s condition?

• Bladder infection
• Urinary tract infection
• Diverticulitis with fistula (Correct answer)
• Appendicitis with fistula
• Prostate cancer with fistula

Correct answer: Diverticulitis with fistula

Left-sided lower abdominal pain and fever are characteristic of diverticulitis. The key symptom here is pneumaturia (passing gas in urine), which indicates a fistula between the bowel and the bladder. Given the location and the CT findings of edema around the sigmoid colon and gas in the bladder, a colovesical fistula secondary to diverticulitis is the most likely diagnosis.

Question 46: An otherwise healthy 27-year-old man is referred to a cardiologist because of three episodes of severe palpitations, dull chest discomfort, and a choking sensation. The episodes occur suddenly and are associated with nausea, faintness, trembling, sweating, and tingling in the extremities; he feels as if he is dying. Within a few hours of each episode, physical examination and laboratory tests show no abnormalities. He does not abuse drugs or alcohol and has no history of interpersonal problems. Which of the following is the most likely diagnosis?

• Delusional disorder
• Panic disorder (Correct answer)
• Generalized anxiety disorder
• Somatization disorder
• Hypochondriasis

Correct answer: Panic disorder

The patient's presentation of recurrent, unexpected episodes characterized by a sudden onset of intense fear or discomfort, accompanied by a cluster of physical symptoms such as palpitations, chest discomfort, choking sensation, nausea, trembling, sweating, and a feeling of impending doom ('as if he is dying'), with no underlying medical cause found, is classic for panic disorder. These attacks are unexpected and lead to significant distress and worry.

Question 47: A psychiatrist's patient discloses a specific, credible plan to kill a named neighbor. Under the Tarasoff ruling, the psychiatrist is legally obligated to:

• Maintain strict confidentiality
• Report the threat to the FBI
• Hospitalize the patient immediately
• Warn the identifiable potential victim (Correct answer)

Correct answer: Warn the identifiable potential victim

The Tarasoff duty-to-warn doctrine requires clinicians to protect identifiable third parties from credible threats.

Question 48: Three days after undergoing elective laparoscopic cholecystectomy for cholelithiasis, a 42-year-old woman has the onset of hematomas at all surgical sites. She was treated for deep venous thrombosis 3 years ago but was not taking any medications at the time of this admission. Results of preoperative laboratory studies were within the reference range. Prior to the operation, she received heparin and underwent application of compression stockings. Her initial postoperative course was uncomplicated. Her only medication is ibuprofen. She is 163 cm (5 ft 4 in) tall and weighs 87 kg (192 lb); BMI is 33 kg/m2 . Her temperature is 37.3°C (99.2°F), pulse is 94/min, respirations are 16/min, and blood pressure is 112/74 mm Hg. Examination shows mild hematomas at all surgical sites. The abdomen is soft and nontender. There is no organomegaly. Bowel sounds are normal. Laboratory studies show: Hemoglobin 10.3 g/dL Hematocrit 30% Leukocyte count 12,00/mm3 Platelet count 45,000/mm3 Which of the following is the most likely cause of this patient’s current findings?

• Inhibition of cyclooxygenase
• Heparin-induced thrombocytopenia (Correct answer)
• Hypersplenism
• Acute intermittent porphyria
• von Willebrand disease

Correct answer: Heparin-induced thrombocytopenia

The patient developed new hematomas and severe thrombocytopenia (platelet count 45,000) three days after receiving heparin, following a history of deep venous thrombosis. This timing and clinical picture are highly suggestive of Heparin-Induced Thrombocytopenia (HIT). HIT is an immune-mediated reaction that causes platelet activation and consumption, leading to thrombocytopenia and a paradoxical prothrombotic state, making it the most likely cause of her current findings.

Question 49: A child with severe combined immunodeficiency (SCID) is found to have adenosine deaminase (ADA) deficiency. Toxic accumulation of which compound destroys lymphocytes?

• Hypoxanthine and xanthine
• Orotic acid
• PRPP
• Deoxyadenosine triphosphate (dATP) (Correct answer)

Correct answer: Deoxyadenosine triphosphate (dATP)

ADA deficiency causes deoxyadenosine accumulation, which is phosphorylated to dATP; excess dATP inhibits ribonucleotide reductase and induces apoptosis in lymphocytes.

Question 50: Which gastric cell type produces both hydrochloric acid and intrinsic factor?

• Chief cells
• Parietal cells (Correct answer)
• G cells
• Mucous neck cells

Correct answer: Parietal cells

Parietal cells secrete HCl via the H+/K+-ATPase pump and also produce intrinsic factor required for vitamin B12 absorption in the terminal ileum.

Question 51: A patient with complete biliary obstruction would most likely develop deficiency of which vitamins?

• Vitamin C and folate
• B-complex vitamins
• Vitamin B12 only
• Fat-soluble vitamins A, D, E, and K (Correct answer)

Correct answer: Fat-soluble vitamins A, D, E, and K

Bile salts are required for micelle formation and absorption of fat-soluble vitamins (A, D, E, K); biliary obstruction prevents this, causing deficiencies and coagulopathy.

Question 52: Which dopaminergic pathway is most associated with the positive symptoms of schizophrenia?

• Mesocortical pathway
• Mesolimbic pathway (Correct answer)
• Tuberoinfundibular pathway
• Nigrostriatal pathway

Correct answer: Mesolimbic pathway

Hyperactivity of the mesolimbic dopaminergic pathway (VTA to nucleus accumbens/limbic areas) underlies the positive symptoms (hallucinations, delusions) of schizophrenia.

Question 53: A 14-year-old boy is brought to the physician for examination prior to participating on his school’s soccer team. Physical examination shows jaundice. Serum studies show a total bilirubin concentration of 2.5 mg/dL, ALT activity of 70 U/L, and ceruloplasmin concentration of 5 mg/dL (N=20–40). A slit-lamp examination shows the presence of brownish rings in the cornea, surrounding the iris. The most appropriate treatment at this time is a drug with which of the following mechanisms of action?

• Increases urine lead excretion (Correct answer)
• Decreases serum bilirubin concentration
• Increases urine copper excretion
• Increases serum chloride concentration

Correct answer: Increases urine lead excretion

The patient's clinical presentation, including jaundice, low ceruloplasmin, and Kayser-Fleischer rings, is highly suggestive of Wilson's disease, a disorder of copper accumulation. While the primary treatment for Wilson's disease involves chelating agents that increase urine copper excretion, some of these agents, such as D-penicillamine, are broad-spectrum chelators. D-penicillamine can also chelate lead, thus increasing its urinary excretion, making this a potential, albeit secondary, mechanism of action for such a drug.

Question 54: Which measure of association is used in a case-control study?

• Incidence rate ratio
• Attributable risk percent
• Odds ratio (Correct answer)
• Relative risk

Correct answer: Odds ratio

Because case-control studies start with outcome status (not exposure), incidence cannot be calculated directly; the odds ratio is used as an approximation of relative risk.

Question 55: A disease has a prevalence of 50 per 1,000 and an incidence of 10 per 1,000 per year. What is the average disease duration?

• 0.2 years
• 10 years
• 50 years
• 5 years (Correct answer)

Correct answer: 5 years

Using the steady-state formula Prevalence = Incidence × Duration: Duration = 50/10 = 5 years.

Question 56: A 24-year-old man is brought by his family with 8 months of social withdrawal, flat affect, disorganized speech, and auditory hallucinations telling him to harm himself. He has no substance use or medical causes identified. What is the most likely diagnosis?

• Schizoaffective disorder
• Bipolar disorder with psychotic features
• Schizophrenia (Correct answer)
• Brief psychotic disorder

Correct answer: Schizophrenia

Schizophrenia is characterized by positive symptoms (hallucinations, delusions), negative symptoms (flat affect, alogia), and functional decline lasting >6 months with no mood episode explanation.

Question 57: A neonate develops respiratory distress shortly after birth. Chest X-ray shows ground-glass opacities bilaterally. Autopsy reveals hyaline membranes lining alveolar ducts. What is the primary deficiency?

• Surfactant (dipalmitoylphosphatidylcholine) (Correct answer)
• Alpha-1 antitrypsin
• Lysyl oxidase
• Fibronectin

Correct answer: Surfactant (dipalmitoylphosphatidylcholine)

Neonatal respiratory distress syndrome results from surfactant deficiency in premature infants, leading to alveolar collapse and hyaline membrane formation.

Question 58: Satellite cells surrounding neuronal cell bodies in dorsal root ganglia are analogous to which CNS glial cell type?

• Oligodendrocytes
• Ependymal cells
• Microglia
• Astrocytes (Correct answer)

Correct answer: Astrocytes

Satellite cells in peripheral ganglia support and regulate the microenvironment of neuronal somas, analogous to astrocyte function in the CNS.

Question 59: A 70-year-old man comes to the physician because of a 2-year history of shortness of breath and progressive chest pain. He worked as a carpenter installing insulation from the ages of 21 to 31 years. Physical examination shows absent breath sounds and dullness to percussion over the right lung base. A chest x-ray shows thickened pleura on the right side and a medium-sized pleural effusion. Pleural fluid studies show a protein concentration greater than 3 g/dL (N=1–2). A pleural biopsy specimen shows spindle cells. Which of the following is the most likely diagnosis?

• Lung abscess
• Viral pneumonia
• Mesothelioma (Correct answer)
• Small cell carcinoma of the lung
• Emphysema

Correct answer: Mesothelioma

The patient's history of asbestos exposure (carpenter installing insulation) combined with progressive shortness of breath, chest pain, thickened pleura, and a pleural effusion is highly suggestive of mesothelioma. The pleural fluid with high protein and the biopsy showing spindle cells are characteristic pathological features of this aggressive malignancy arising from the pleura, which is strongly linked to asbestos exposure.

Question 60: A 17-year-old girl has cyclic pelvic pain each month but has never had menstrual flow. She has normal breast development and pubic hair. Exam reveals a bluish, tense, bulging membrane at the vaginal introitus. What is the diagnosis?

• Vaginal agenesis (MRKH syndrome)
• Androgen insensitivity syndrome
• Imperforate hymen with hematocolpos (Correct answer)
• Transverse vaginal septum

Correct answer: Imperforate hymen with hematocolpos

Imperforate hymen causes hematocolpos (accumulated menstrual blood), presenting as cyclic pain without external bleeding and a characteristic bluish bulging membrane at the introitus.

Question 61: A hepatitis virus has a partially double-stranded circular DNA genome and replicates via an RNA intermediate using a viral reverse transcriptase. Which hepatitis virus has this replication strategy?

• Hepatitis C virus
• Hepatitis A virus
• Hepatitis B virus (Correct answer)
• Hepatitis D virus

Correct answer: Hepatitis B virus

Hepatitis B virus (HBV) is a hepadnavirus with a partially dsDNA genome; it replicates through an RNA pregenome intermediate using reverse transcriptase, making it targetable by nucleoside analogs.

Question 62: A 45-year-old executive has a pervasive pattern of grandiosity, requires excessive admiration, lacks empathy, and reacts to criticism with rage. Which personality disorder best describes him?

• Obsessive-compulsive personality disorder
• Narcissistic personality disorder (Correct answer)
• Antisocial personality disorder
• Histrionic personality disorder

Correct answer: Narcissistic personality disorder

Narcissistic personality disorder is characterized by grandiosity, need for admiration, lack of empathy, and a sense of entitlement.

Question 63: A 19-year-old woman comes to the physician because of a 3-month history of intermittent drooping of her left eyelid each evening and occasional difficulty chewing and swallowing. She also has had two episodes of double vision that occurred in the evening and resolved by the following morning. Examination shows no abnormalities except for slight ptosis on the right. Which of the following is the most likely diagnosis?

• Guillain-Barré syndrome
• Myasthenia gravis (Correct answer)
• Brain stem glioma
• Complex partial seizures
• Acute intermittent porphyria

Correct answer: Myasthenia gravis

The patient's fluctuating symptoms of ptosis, diplopia, and difficulty chewing/swallowing, which worsen with activity or at the end of the day and improve with rest, are highly characteristic of myasthenia gravis. This autoimmune disorder involves antibodies attacking acetylcholine receptors at the neuromuscular junction, leading to fatigable muscle weakness, particularly affecting ocular and bulbar muscles.

Question 64: A previously healthy 67-year-old woman comes to the physician with her husband because of a 4-month history of a resting tremor of her right arm. Her husband reports that her movements have been slower and that she appears less stable while walking. Examination shows increased muscle tone in the upper extremities that is greater on the right than on the left. There is decreased right arm swing. Her gait is slow and shuffling. Which of the following is the most likely explanation for this patient’s symptoms?

• Excessive output of oxytocin
• Excessive thalamic output of norepinephrine
• Decreased dopaminergic input to the striatum (Correct answer)
• Decreased serotonergic activity in the brain stem
• Bilateral frontal lobe degeneration

Correct answer: Decreased dopaminergic input to the striatum

The patient's symptoms—resting tremor, slowed movements (bradykinesia), unstable gait (postural instability), increased muscle tone (rigidity), and decreased arm swing—are classic manifestations of Parkinson's disease. This neurodegenerative disorder is primarily caused by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta, leading to a significant reduction in dopamine input to the striatum and subsequent disruption of basal ganglia function.

Question 65: Which connective tissue is characterized by irregularly arranged collagen bundles and is found in the dermis of the skin?

• Loose areolar connective tissue
• Dense irregular connective tissue (Correct answer)
• Dense regular connective tissue
• Elastic connective tissue

Correct answer: Dense irregular connective tissue

Dense irregular connective tissue has randomly oriented collagen bundles that resist tension from multiple directions, as in the dermis.

Question 66: Which beta-blocker property makes carvedilol preferable over metoprolol in patients with heart failure and peripheral vascular disease?

• Intrinsic sympathomimetic activity
• Cardioselectivity for beta-1 receptors
• Alpha-1 blocking activity causing vasodilation (Correct answer)
• Membrane-stabilizing activity

Correct answer: Alpha-1 blocking activity causing vasodilation

Carvedilol blocks alpha-1 receptors in addition to beta receptors, causing peripheral vasodilation that benefits patients with peripheral vascular disease.

Question 67: A 72-year-old woman develops acute confusion, disorientation, and visual hallucinations 2 days after hip replacement surgery. Her symptoms fluctuate throughout the day. Which diagnosis is most likely?

• Vascular dementia
• Brief psychotic disorder
• Lewy body dementia
• Delirium (Correct answer)

Correct answer: Delirium

Delirium is characterized by acute onset, fluctuating course, inattention, and disturbance of consciousness, commonly occurring postoperatively.

Question 68: A 28-year-old woman has two episodes of optic neuritis and a spinal cord lesion separated by time and space on MRI. CSF shows oligoclonal bands. Which diagnostic criteria confirm MS?

• Miller Fisher criteria
• CHADS2 criteria
• McDonald criteria (2017) (Correct answer)
• Poser criteria

Correct answer: McDonald criteria (2017)

The 2017 McDonald criteria require dissemination in space (DIS) and time (DIT) demonstrated by MRI or clinical events to diagnose MS.

Question 69: A 60-year-old man has tenderness in the region distally between the tendons of the extensor pollicis longus and extensor pollicis brevis (anatomical snuffbox) after falling on the palm of his right hand. A fracture of which of the following carpal bones is most likely in this patient?

• Trapezium
• Hamate
• Lunate
• Triquetrum
• Scaphoid (Correct answer)

Correct answer: Scaphoid

A fall on an outstretched hand (FOOSH injury) is a common mechanism for carpal bone fractures. Tenderness specifically in the anatomical snuffbox, located between the tendons of the extensor pollicis longus and extensor pollicis brevis, is the classic clinical sign of a scaphoid fracture. Scaphoid fractures are particularly concerning due to their risk of avascular necrosis.

Question 70: A 47-year-old woman comes to the physician for a routine health maintenance examination. She feels well and has no history of serious illness. Her mother, brother, and sister have hypertension. The patient’s pulse is 84/min, and blood pressure is 138/85 mm Hg. Examination shows no other abnormalities. The most appropriate recommendation is decreased intake of which of the following?

• Carbohydrates
• Calcium
• Protein
• Sodium (Correct answer)
• Potassium

Correct answer: Sodium

This patient has elevated blood pressure (138/85 mm Hg), which is considered elevated blood pressure or pre-hypertension, and a family history of hypertension. Dietary sodium restriction is a primary and highly effective lifestyle modification recommended for preventing and managing hypertension. Reducing sodium intake helps lower blood pressure and is a crucial first step in patients with elevated readings or a family history of the condition.

Question 71: A 40-year-old man undergoes total thyroidectomy for papillary thyroid cancer. Six hours postoperatively he complains of tingling around his mouth and in his fingertips. His calcium level is 7.2 mg/dL. What is the immediate treatment?

• IV magnesium sulfate
• Oral calcium carbonate supplementation
• Parathyroid reimplantation
• IV calcium gluconate (Correct answer)

Correct answer: IV calcium gluconate

Symptomatic hypocalcemia after thyroidectomy requires immediate IV calcium gluconate to prevent laryngospasm and tetany.

Question 72: A 37-year-old man with type 1 diabetes mellitus comes to the physician for a routine examination. His only medication is insulin. His pulse is 72/min, respirations are 12/min, and blood pressure is 138/88 mm Hg. Funduscopic examination shows microaneurysms and hemorrhages. Sensation to vibration and light touch is decreased over the lower extremities. His serum creatinine concentration is 1.6 mg/dL. A 24-hour urine collection shows 550 mg of protein. Treatment with which of the following is most likely to slow progression of this patient’s renal disease?

• Lisinopril (Correct answer)
• Hydralazine
• Hydrochlorothiazide
• Atenolol
• Clonidine

Correct answer: Lisinopril

The patient has evidence of diabetic nephropathy, characterized by proteinuria and elevated serum creatinine. Angiotensin-converting enzyme (ACE) inhibitors, such as lisinopril, are the cornerstone of treatment for diabetic nephropathy. They reduce intraglomerular pressure by dilating the efferent renal arteriole, thereby decreasing proteinuria and significantly slowing the progression of renal disease.

Question 73: A 3-week-old infant is brought to the physician by his mother because of a 1-week history of increasingly frequent vomiting. She says that at first he vomited occasionally, but now he vomits after every feeding. The vomitus is nonbilious and consists of breast milk. He has had fewer wet diapers during the past 2 days. He was born at term following an uncomplicated pregnancy and initially fed well. He appears lethargic and dehydrated. A 1 × 2-cm, firm, mobile, olive-shaped mass is palpated immediately to the left of the epigastrium. Which of the following is the most likely diagnosis?

• Congenital megacolon (Hirschsprung disease)
• Midgut volvulus
• Pyloric stenosis
• Intussusception
• Duodenal atresia

The infant's presentation of progressive, nonbilious vomiting after every feeding, dehydration, and a palpable 'olive-shaped' mass in the epigastrium is classic for pyloric stenosis. This condition involves hypertrophy of the pyloric muscle, which obstructs gastric outflow. The nonbilious nature of the vomitus indicates the obstruction is proximal to the ampulla of Vater, preventing bile from entering the stomach.

Question 74: A 35-year-old man is found unresponsive. RR is 5 breaths/min, pupils are pinpoint bilaterally, and he is cyanotic. Bystanders report he uses opioids. What is the immediate treatment?

• IV flumazenil 0.2 mg
• Bag-mask ventilation alone
• IV naloxone 0.4–2 mg (Correct answer)
• Activated charcoal via nasogastric tube

Correct answer: IV naloxone 0.4–2 mg

The opioid toxidrome (miosis, respiratory depression, unconsciousness) requires immediate IV naloxone, a competitive opioid receptor antagonist.

Question 75: A dog that was conditioned to salivate at a bell tone stops salivating after the bell is repeatedly presented without food. This process is called:

• Habituation
• Extinction (Correct answer)
• Discrimination
• Generalization

Correct answer: Extinction

Extinction occurs when a conditioned response diminishes because the conditioned stimulus is no longer paired with the unconditioned stimulus.

Question 76: Xeroderma pigmentosum presents with extreme UV sensitivity and high skin cancer risk. Which DNA repair pathway is defective?

• Non-homologous end joining (NHEJ)
• Base excision repair (BER)
• Nucleotide excision repair (NER) (Correct answer)
• Mismatch repair (MMR)

Correct answer: Nucleotide excision repair (NER)

NER removes bulky helix-distorting lesions such as pyrimidine dimers caused by UV radiation; XP mutations in NER endonucleases allow these lesions to persist.

Question 77: A 42-year-old woman, gravida 2, para 2, comes to the physician because of increasingly frequent loss of urine during the past year. She has loss of urine when she coughs, sneezes, exercises, or plays with her children. Her incontinence is never preceded by a sudden urge to void, and she does not have loss of urine at night. Her children were born after uncomplicated vaginal deliveries. She has no history of other hospital admissions or serious illness. She takes no medications. Abdominal examination shows no abnormalities. The external genitalia, vagina, and cervix appear normal. The uterus and adnexa are normal to palpation. There is loss of a small amount of urine with Valsalva maneuver. Her postvoid residual volume is 50 mL. Urinalysis shows no abnormalities. Which of the following is the most likely diagnosis?

• Urinary tract infection
• Stress incontinence (Correct answer)
• Overflow incontinence
• Vesicovaginal fistula
• Overactive bladder with incontinence

Correct answer: Stress incontinence

The patient's symptoms of involuntary urine leakage with activities that increase intra-abdominal pressure (coughing, sneezing, exercise) and the absence of urgency or nocturnal incontinence are classic for stress incontinence. The finding of urine loss with a Valsalva maneuver and a normal postvoid residual volume further supports this diagnosis, indicating urethral hypermobility or intrinsic sphincter deficiency.

Question 78: A 7-year-old boy fails to develop spoken language, avoids eye contact, and engages in repetitive rocking movements. He was hitting all developmental milestones until age 18 months. What is the most likely diagnosis?

• Selective mutism
• Fragile X syndrome
• Autism spectrum disorder (Correct answer)
• Rett syndrome

Correct answer: Autism spectrum disorder

Autism spectrum disorder presents with deficits in social communication and restricted/repetitive behaviors; early development may appear normal before regression.

Question 79: A 52-year-old woman with glioblastoma multiforme in the frontal lobe tells her physician that she does not want operative treatment. She is mentally competent and understands that an operation is the only effective treatment of her tumor, and that without an operation she will die. She is afraid of the adverse effects of an operation and says she has lived a long and happy life. Two weeks later, she lapses into a coma, and her husband requests that the operation be carried out. Which of the following is the most appropriate consideration for her physician in deciding whether to operate?

• Her previously expressed wishes (Correct answer)
• Medical indications for treatment
• Her husband’s request
• Her quality of life
• Her age

Correct answer: Her previously expressed wishes

This scenario highlights the ethical principle of patient autonomy and the importance of respecting previously expressed wishes. When a mentally competent patient makes an informed decision about their medical care, that decision remains valid even if they later lose capacity. The physician must honor the patient's clear refusal of surgery, despite the husband's subsequent request.

Question 80: A researcher compares mean systolic blood pressure between a drug group and a placebo group (continuous outcome, two independent groups, normally distributed). Which statistical test is MOST appropriate?

• Chi-squared test
• Kaplan-Meier analysis
• Student's t-test (Correct answer)
• Pearson correlation

Correct answer: Student's t-test

The Student's t-test is used to compare means between two independent groups when data are normally distributed.

Question 81: A 32-year-old man comes to the physician because of a 12-day history of abdominal cramps and bloating, diarrhea, and flatulence. He says that he started a new exercise program 2 weeks ago and has been consuming a high quantity of yogurt bars, peanut butter, and protein- and calorie-enriched milk shakes to “bulk up.” He has no history of serious illness and takes no medications. His temperature is 37°C (98.6°F). The abdomen is distended, nontender, and tympanitic to percussion. Bowel sounds are increased. The remainder of the examination shows no abnormalities. Which of the following is the most likely cause of this patient’s symptoms?

• Fungal overgrowth in the small bowel
• Incarcerated hernia
• Allergy to peanuts
• Lactase deficiency (Correct answer)
• Irritable bowel syndrome

Correct answer: Lactase deficiency

The patient's symptoms of abdominal cramps, bloating, diarrhea, and flatulence, which began after increasing his consumption of dairy-containing products like yogurt bars and milkshakes, are highly suggestive of lactase deficiency (lactose intolerance). This condition results from the inability to digest lactose, leading to its fermentation by gut bacteria and the production of gas and osmotic diarrhea.

Question 82: A 34-year-old woman is brought to the emergency department by her husband because of confusion for 2 hours. She is unconscious on arrival. Her husband says that she has a 1-year history of episodes of nervousness, light-headedness, and dizziness that resolve after she eats a meal. Physical examination shows no other abnormalities. Her serum glucose concentration is 25 mg/dL. After an intravenous infusion of 0.9% saline and glucose, she regains consciousness. Which of the following is the most likely diagnosis?

• Multiple endocrine neoplasia syndrome
• Pheochromocytoma
• Pancreatic gastrinoma
• Cushing syndrome
• Insulinoma (Correct answer)

Correct answer: Insulinoma

The patient's presentation of recurrent episodes of neuroglycopenic symptoms (confusion, unconsciousness, nervousness, light-headedness) that resolve with food intake, coupled with a critically low serum glucose (25 mg/dL), is classic for an insulinoma. An insulinoma is a tumor of the pancreatic beta cells that secretes excessive insulin, leading to profound hypoglycemia. The resolution of symptoms with glucose administration further supports this diagnosis.

Question 83: Hypospadias results from failure of fusion of which embryonic structures?

• Urorectal septum
• Labioscrotal swellings
• Urogenital folds (Correct answer)
• Genital tubercle

Correct answer: Urogenital folds

The urethral (urogenital) folds fuse in the midline to enclose the urethral plate and form the penile urethra; failure of fusion results in hypospadias with the urethral meatus on the ventral surface.

Question 84: A 15-year-old girl is brought to the physician by her mother because of a 1-year history of monthly cramps that begin 2 days before menses and last 3 days. The cramps have increased in severity during the past 6 months. She is unable to practice with her volleyball team because of the pain and typically misses 2 days of school monthly. Menarche was at the age of 12 years. Menses occur at regular 30-day intervals and last 4 days with normal flow. She has no history of serious illness and takes no medications. She has never been sexually active. Physical examination shows no abnormalities. Which of the following is the most likely cause of this patient’s pain?

• Endometrial prostaglandin production (Correct answer)
• Degenerating myoma
• Bladder wall permeability
• Escherichia coli infection
• Intestinal hypermobility

Correct answer: Endometrial prostaglandin production

This patient's symptoms are characteristic of primary dysmenorrhea, which is painful menstruation without an identifiable pelvic pathology. The pain is primarily caused by the excessive production and release of prostaglandins from the endometrium during the menstrual cycle. These prostaglandins induce strong uterine contractions, leading to uterine ischemia and the sensation of cramping pain.

Question 85: A 13-year-old girl is brought to the physician because of a 1-year history of intermittent irregular vaginal bleeding; the bleeding ranges from spotting to heavier than a normal menstrual period, occurs every 2 to 8 weeks, and lasts 10 to 30 days. Examination shows a pink, well-rugated vagina with no discharge; the cervix appears normal. The uterus is 6 cm in length, regular in contour, and nontender. There are palpable, normal-sized, nontender ovaries. Which of the following is the most appropriate pharmacotherapy to alleviate this patient’s symptoms?

• Continuous low-dose estrogen
• Thyroid hormone
• Gonadotropin-releasing hormone agonist
• Oral contraceptives (Correct answer)
• Tetracycline

Correct answer: Oral contraceptives

Irregular and heavy menstrual bleeding in an adolescent, especially within the first few years post-menarche, is commonly due to anovulatory cycles, leading to dysfunctional uterine bleeding. Oral contraceptives (OCPs) are the most appropriate pharmacotherapy as they regulate the menstrual cycle, reduce bleeding, and provide contraception by stabilizing the endometrial lining and inducing regular withdrawal bleeds.

Question 86: Which enzyme is unique to gluconeogenesis and is not used in glycolysis?

• Enolase
• Phosphoglycerate kinase
• Phosphoenolpyruvate carboxykinase (PEPCK) (Correct answer)
• Phosphoglycerate mutase

Correct answer: Phosphoenolpyruvate carboxykinase (PEPCK)

PEPCK converts oxaloacetate to PEP, bypassing the irreversible pyruvate kinase reaction of glycolysis; it is exclusive to gluconeogenesis.

Question 87: A 52-year-old man comes to the physician because of a 5-month history of pain in his left knee that is exacerbated by walking long distances. There is no history of trauma. He has hypertension well controlled with enalapril. His job does not require carrying heavy loads. He is 180 cm (5 ft 11 in) tall and weighs 95 kg (210 lb); BMI is 29 kg/m2 . His pulse is 82/min and regular, respirations are 16/min, and blood pressure is 130/82 mm Hg. Examination of the left knee shows mild crepitus with flexion and extension; there is no effusion or warmth. X-rays of the knees show narrowing of the joint space in the left knee compared with the right knee. Which of the following is most likely to improve this patient’s symptoms?

• Daily vitamin D supplementation
• High-impact aerobics program
• Weight loss program (Correct answer)
• Daily colchicine prophylaxis
• Daily shark cartilage supplementation

Correct answer: Weight loss program

The patient's knee pain exacerbated by walking, crepitus, and joint space narrowing on X-ray are consistent with osteoarthritis. His BMI of 29 kg/m2 indicates he is overweight, which significantly increases the mechanical load on his weight-bearing knee joints. Reducing this load through a weight loss program is a highly effective intervention to decrease pain, improve joint function, and slow the progression of osteoarthritis.

Question 88: A 29-year-old man presents with recurrent, unexpected panic attacks with palpitations, shortness of breath, and fear of dying, followed by 1 month of worry about future attacks and behavioral avoidance. What is the diagnosis?

• Generalized anxiety disorder
• Agoraphobia alone
• Panic disorder (Correct answer)
• Specific phobia

Correct answer: Panic disorder

Panic disorder requires recurrent unexpected panic attacks plus persistent concern about future attacks or maladaptive behavioral changes lasting at least 1 month.

Question 89: A 65-year-old man undergoes a low anterior resection for rectal cancer. Three days post-op he develops fever, tachycardia, leukocytosis, and purulent output from a pelvic drain. CT confirms an anastomotic leak with contained pelvic abscess. He is hemodynamically stable. What is the appropriate management?

• CT-guided drainage, IV antibiotics, and proximal diversion if needed (Correct answer)
• Immediate re-laparotomy and Hartmann's procedure
• IV antibiotics alone
• Colonoscopic placement of a covered stent

Correct answer: CT-guided drainage, IV antibiotics, and proximal diversion if needed

A contained anastomotic leak in a stable patient is managed with CT-guided drainage, IV antibiotics, and fecal diversion if the anastomosis cannot be salvaged.

Question 90: A 50-year-old man with type 2 diabetes mellitus has a 1-week history of swelling and a feeling “like electric shocks” in his right wrist and hand. He is a computer programmer, and the shock-like feeling is worse with activity and at the end of the day. There is atrophy of the right thenar eminence. With the hand hyperextended, pain radiates into the fingers when the examiner taps the flexor surface of the distal wrist. Which of the following nerves is most likely compressed in this patient?

• Superficial radial
• Musculocutaneous
• Radial ulnar
• Deep radial
• Median (Correct answer)

Correct answer: Median

The symptoms of 'electric shocks' (paresthesias) radiating into the fingers, worsening with activity, thenar eminence atrophy, and a positive Tinel's sign at the wrist are classic indicators of carpal tunnel syndrome. This condition involves compression of the median nerve as it passes through the carpal tunnel in the wrist, often exacerbated by repetitive hand movements and common in patients with diabetes due to nerve susceptibility.

Question 91: A 4-year-old presents with periorbital edema, frothy urine, and hypoalbuminemia. Urine dipstick shows 4+ protein with no blood. Most likely diagnosis?

• IgA nephropathy
• Focal segmental glomerulosclerosis
• Minimal change disease (Correct answer)
• Post-streptococcal GN

Correct answer: Minimal change disease

Minimal change disease is the most common cause of nephrotic syndrome in children aged 1–8 and responds to steroids.

Question 92: An 8-month-old girl is brought to the emergency department because of a 1-day history of rapid breathing. Her temperature is 38.4°C (101.1°F), pulse is 160/min, respirations are 60/min, and blood pressure is 100/68 mm Hg. Bilateral expiratory wheezing and crackles are heard on auscultation. A chest x-ray shows areas of atelectasis and hyperinflation. Which of the following is the most likely causal virus?

• Respiratory syncytial virus (Correct answer)
• Rhinovirus
• Varicella-zoster virus
• Measles virus

Correct answer: Respiratory syncytial virus

The clinical picture of an 8-month-old infant with rapid breathing, fever, bilateral expiratory wheezing and crackles, and chest x-ray findings of atelectasis and hyperinflation is highly characteristic of bronchiolitis. Respiratory syncytial virus (RSV) is the most common cause of bronchiolitis in infants and young children. RSV infection leads to inflammation and obstruction of the small airways, resulting in the observed respiratory distress and lung findings.

Question 93: In the urea cycle, one nitrogen is derived from free ammonia and the other is donated by which compound?

• Alanine
• Aspartate (Correct answer)
• Glycine
• Glutamine

Correct answer: Aspartate

The two urea nitrogens originate from ammonium (via carbamoyl phosphate synthetase I) and from aspartate (incorporated at the argininosuccinate synthetase step).

Question 94: Drug X applied to a nerve axon decreases the duration of the action potential without affecting the resting potential or peak amplitude of the action potential. Which of the following is the most likely mechanism of action of Drug X?

• Increase in the rate of voltage-dependent changes in K + permeability (Correct answer)
• Block of voltage-dependent Na + permeability
• Decrease in the rate of Na + inactivation
• Decrease in voltage-dependent Na + permeability
• Inhibition of the Na + –K + pump

Correct answer: Increase in the rate of voltage-dependent changes in K + permeability

The action potential's duration is primarily determined by the repolarization phase, which is mediated by the efflux of potassium ions through voltage-gated K+ channels. An increase in the rate of voltage-dependent changes in K+ permeability means these channels open faster or more extensively, accelerating K+ efflux and thus shortening the repolarization time and the overall action potential duration. Options affecting Na+ permeability (A, B, C) would alter depolarization or peak amplitude, which are stated to be unaffected, while inhibition of the Na+-K+ pump (E) affects resting potential maintenance, not action potential duration.

Question 95: 77-year-old man comes to the physician with his daughter for a follow-up examination to learn the results of neuropsychological testing performed 1 week ago for evaluation of a recent memory loss. Results of the testing indicated cognitive changes consistent with early stages of dementia. Three weeks ago, he was diagnosed with prostate cancer and has shown signs of a depressed mood since then. Twenty years ago, he required treatment in a hospital for major depressive disorder. His symptoms resolved with antidepressant therapy, and he has not taken any psychotropic medication for the past 15 years. The patient’s daughter comes into the examination room before her father and asks that the physician not tell her father any information that might be upsetting, given his vulnerability to depression. She says she is concerned about what the results might be and how her father will handle them. The patient enters the room soon after his daughter makes her request. Which of the following is the most appropriate initial physician statement to this patient?

• “Before going over your test results, I’d like to hear how you have been doing. You have been through a difficult time.” (Correct answer)
• “I would like to talk with your daughter a bit about your test results, and then I will go over things with you.”
• “Because of your history of depression, I would like you to start on an antidepressant medication before we talk any further about your neuropsychological testing.”
• “Your tests were inconclusive, and I would like to have you start on a medication to help with your memory as a precaution.”
• “Your daughter is concerned about you. I think you need to see a psychiatrist before we go any further here.”

Correct answer: “Before going over your test results, I’d like to hear how you have been doing. You have been through a difficult time.”

This response prioritizes the patient's autonomy and right to receive information directly, while also acknowledging his recent stressors (prostate cancer, memory loss) and emotional vulnerability. It opens a dialogue, allowing the physician to assess his current emotional state and readiness to receive the information, rather than immediately disclosing or withholding it based on the daughter's request. This approach respects the patient-physician relationship and prepares the patient for potentially difficult news.

Question 96: A Gram-negative organism isolated from a catheter-associated UTI produces an enzyme that inactivates aminoglycosides by covalent modification. Which type of enzyme is responsible?

• Methyltransferase modifying ribosomal RNA
• DNA gyrase mutation
• Aminoglycoside acetyltransferase/phosphotransferase (Correct answer)
• Beta-lactamase

Correct answer: Aminoglycoside acetyltransferase/phosphotransferase

Aminoglycoside resistance most commonly results from transferase enzymes (acetyltransferases, phosphotransferases, nucleotidyltransferases) that chemically modify the drug and prevent it from binding the 30S ribosome.

Question 97: Which of the following is an example of PRIMARY active transport?

• Na+/H+ exchanger
• SGLT1 (sodium-glucose cotransporter)
• Na+/K+-ATPase (Correct answer)
• GLUT4

Correct answer: Na+/K+-ATPase

Na+/K+-ATPase directly hydrolyzes ATP to move 3 Na+ out and 2 K+ in against their concentration gradients — the definition of primary active transport.

Question 98: A 68-year-old man undergoes elective CABG. On postoperative day 3, his urine output drops to 15 mL/hr, creatinine rises from 1.0 to 2.8 mg/dL, and he remains hemodynamically stable. What is the most likely diagnosis?

• Contrast nephropathy
• Acute tubular necrosis (Correct answer)
• Pre-renal azotemia from hypovolemia
• Urinary tract obstruction

Correct answer: Acute tubular necrosis

Acute kidney injury peaking 48–72 hours after major cardiac surgery most commonly represents ischemic acute tubular necrosis from intraoperative hypoperfusion.

Question 99: A 29-year-old man who emigrated from Scotland 3 years ago is brought to the emergency department because of severe shortness of breath for 2 hours. He has a debilitating condition that began 2 years ago with an odd sticky feeling of his skin, but physical examination at that time showed no abnormalities. His condition has progressed to include severe major depressive disorder, dementia, unsteady gait, difficulty walking, and impaired coordination. He now is confined to a wheelchair and has severe dementia and the inability to speak. There is no family history of a similar condition. On arrival, his respirations are 24/min, and he dies shortly thereafter. At autopsy, examination of the brain shows a spongiform encephalopathy. Which of the following is the most likely diagnosis?

• Variant Creutzfeldt-Jakob disease (Correct answer)
• Parkinson disease
• Dementia, Alzheimer type
• Pick disease
• Frontotemporal dementia

Correct answer: Variant Creutzfeldt-Jakob disease

The patient's young age, rapid progression of psychiatric symptoms (severe depression), sensory complaints ('sticky feeling'), neurological decline (dementia, ataxia), and spongiform encephalopathy on autopsy are highly characteristic of variant Creutzfeldt-Jakob disease (vCJD). His emigration from Scotland is also relevant, as vCJD is linked to the consumption of beef products contaminated with bovine spongiform encephalopathy (BSE) prions, which was prevalent in the UK.

Question 100: A 22-year-old college student develops fever, pharyngitis, and posterior cervical lymphadenopathy. A peripheral blood smear shows atypical lymphocytes. The heterophile antibody test is positive. Which cell type is the primary reservoir for the causative virus?

• T lymphocytes
• Epithelial cells
• B lymphocytes (Correct answer)
• Monocytes

Correct answer: B lymphocytes

Epstein-Barr virus (EBV) infects B lymphocytes via CD21 (complement receptor 2); the atypical lymphocytes seen on smear are reactive cytotoxic T cells responding to infected B cells.

Question 101: A 36-year-old woman with hypertension well-controlled on lisinopril asks about contraception. Which method is CONTRAINDICATED in this patient?

• Progestin-only pills (mini-pill)
• Combined estrogen-progestin oral contraceptives (Correct answer)
• Copper intrauterine device
• Levonorgestrel-releasing IUD

Correct answer: Combined estrogen-progestin oral contraceptives

Combined oral contraceptives are contraindicated in women with hypertension due to increased risk of stroke and myocardial infarction, especially over age 35.

Question 102: A 10-month-old male has recurrent bacterial infections including pneumonia and otitis media since 6 months of age. Serum immunoglobulins are markedly low. B-cell count is undetectable. Most likely diagnosis?

• SCID
• Selective IgA deficiency
• X-linked agammaglobulinemia (Bruton) (Correct answer)
• Hyper-IgM syndrome

Correct answer: X-linked agammaglobulinemia (Bruton)

X-linked agammaglobulinemia (Bruton) presents with absent B cells and profound hypogammaglobulinemia, causing recurrent bacterial infections after maternal antibody wanes at 6 months.

Question 103: A 60-year-old diabetic presents for routine labs. eGFR is 28 mL/min/1.73m², K+ is 5.6 mEq/L, HCO3 is 18 mEq/L, and phosphorus is elevated. Which medication requires dose reduction or avoidance in this patient?

• Amlodipine
• Atorvastatin
• Lisinopril
• Metformin (Correct answer)

Correct answer: Metformin

Metformin is contraindicated when eGFR falls below 30 mL/min/1.73m² due to risk of lactic acidosis from impaired renal clearance.

Question 104: A 22-year-old woman comes to the physician because of a 10-day history of pain in multiple joints. She first had pain in her right elbow, and then her right shoulder, and now has pain, redness, and swelling in her left knee that began 2 days ago. She currently has no pain in the right shoulder and elbow. There is no history of trauma. She is sexually active, and she and her partner use condoms for contraception inconsistently. Examination of the left knee shows warmth, erythema, tenderness, and soft-tissue swelling. Range of motion of the knee is limited to 10 degrees of flexion. The remainder of the examination, including pelvic examination, shows no abnormalities. Arthrocentesis of the knee joint yields 10 mL of cloudy fluid with a leukocyte count of 18,300/mm3 (97% segmented neutrophils). Microscopic examination of the leukocytes within the joint fluid is most likely to show which of the following?

• Gram-negative diplococci
• Acid-fast bacteria
• Cuboidal positively birefringent crystals
• Needle-shaped negatively birefringent crystals
• Gram-positive cocci in clusters (Correct answer)

Correct answer: Gram-positive cocci in clusters

The patient's migratory polyarthralgia followed by acute monoarthritis in a young, sexually active woman with inconsistent condom use strongly suggests disseminated gonococcal infection. However, the question asks for the *most likely* microscopic finding in the joint fluid. While Neisseria gonorrhoeae (gram-negative diplococci) is a strong clinical suspicion, Staphylococcus aureus (gram-positive cocci in clusters) remains the most common cause of septic arthritis overall, even in this demographic. Therefore, gram-positive cocci in clusters are the most likely finding on Gram stain of the synovial fluid.

Question 105: A 27-year-old woman comes to the physician because of a 3-week history of fatigue and blurred vision. She occasionally has had double vision during this period. For the past year, she has had 3- to 4-day episodes of numbness and tingling of her arms and legs. She has no personal or family history of serious illness. Her only medication is an oral contraceptive. She has smoked one pack of cigarettes daily for 5 years. Vital signs are within normal limits. Funduscopic examination shows no abnormalities. Visual acuity is decreased in the left eye. Sensation to light touch is decreased over the hands and feet; sensation to pinprick is increased over the fingers and toes bilaterally. An MRI of the brain shows several hyperintense oval plaques in the periventricular region on T2-weighted images. Which of the following is the most likely diagnosis?

• Systemic lupus erythematosus
• Multiple sclerosis (Correct answer)
• Glioma
• Toxoplasmosis
• Guillain-Barré syndrome

Correct answer: Multiple sclerosis

The patient's recurrent neurological symptoms, such as fatigue, blurred vision, double vision, and episodes of numbness/tingling in different areas (disseminated in time and space), are highly suggestive of multiple sclerosis. The MRI findings of hyperintense oval plaques in the periventricular region on T2-weighted images are characteristic demyelinating lesions seen in MS. This combination of clinical presentation and imaging findings strongly supports the diagnosis.

Question 106: A new drug, Drug X, relieves pain by interacting with a specific receptor in the body. Drug X binds irreversibly to this receptor, resulting in a long duration of action. Which of the following types of bonds is most likely formed between Drug X and its receptor?

• van der Waals
• Ionic
• Hydrogen
• Covalent (Correct answer)
• Hydrophobic

Correct answer: Covalent

Irreversible binding of a drug to its receptor typically involves the formation of a strong, stable covalent bond. Unlike weaker non-covalent interactions, covalent bonds require the synthesis of new receptor molecules to restore function. This leads to a prolonged duration of action that is independent of the drug's plasma half-life, as the drug-receptor complex is not easily dissociated.

Question 107: A 65-year-old man with CKD stage 4 is found to have K+ of 6.8 mEq/L on routine labs. ECG shows peaked T waves and widened QRS. Which intervention should be administered first?

• Calcium gluconate IV (Correct answer)
• Furosemide IV
• Sodium bicarbonate IV
• Kayexalate (sodium polystyrene sulfonate)

Correct answer: Calcium gluconate IV

Calcium gluconate IV is given first to stabilize the cardiac membrane and prevent fatal arrhythmias; it acts within minutes and is essential when ECG changes are present.

Question 108: A 6-year-old boy repeatedly wets the bed at night despite normal development. What is the first-line behavioral intervention for primary enuresis?

• Desmopressin at bedtime
• Imipramine
• Urine alarm (bell-and-pad) therapy (Correct answer)
• Oxybutynin

Correct answer: Urine alarm (bell-and-pad) therapy

The urine (enuresis) alarm has the highest long-term cure rate and is first-line behavioral treatment for primary nocturnal enuresis.

Question 109: A previously healthy 15-year-old girl comes to the physician because of increasing left ear pain during the past 3 days. Vital signs are within normal limits. Examination of the left ear shows edema and erythema of the auditory canal with a greenish discharge. Manipulation of the left pinna elicits pain. The tympanic membrane can only partially be visualized, and examination of what can be seen appears normal with normal mobility. Examination of the right ear shows no abnormalities. Which of the following is the most likely diagnosis?

• Acute otitis media
• Furunculosis
• Otitis externa (Correct answer)
• Mastoiditis
• Tympanic membrane perforation

Correct answer: Otitis externa

The patient's symptoms of ear pain, edema and erythema of the auditory canal, greenish discharge, and pain elicited by manipulation of the pinna are classic signs of otitis externa, also known as 'swimmer's ear.' This infection or inflammation of the external auditory canal typically spares the tympanic membrane, which appears normal or only partially visualized due to canal swelling.

Question 110: A screening test has 90% sensitivity and 80% specificity. The disease prevalence is 10%. What is the approximate positive predictive value (PPV)?

• 33% (Correct answer)
• 80%
• 50%
• 90%

Correct answer: 33%

PPV = (sensitivity × prevalence) / [(sensitivity × prevalence) + (1−specificity)(1−prevalence)] = (0.9×0.1) / (0.9×0.1 + 0.2×0.9) ≈ 33%.

Question 111: Failure to bind LDL to its receptor results in uncontrolled synthesis of cholesterol. This occurs because synthesis of which of the following enzymes is not repressed?

• 3-Hydroxy-3-methylglutaryl (HMG)-CoA reductase (Correct answer)
• Lecithin-cholesterol acyltransferase
• Lipoprotein lipase
• Acyl CoA cholesterol acyltransferase
• Lysosomal protease

Correct answer: 3-Hydroxy-3-methylglutaryl (HMG)-CoA reductase

When LDL binding and uptake are impaired, cells perceive a lack of intracellular cholesterol, leading to increased *de novo* cholesterol synthesis. 3-Hydroxy-3-methylglutaryl (HMG)-CoA reductase is the rate-limiting enzyme in cholesterol biosynthesis. Its activity is normally repressed by high intracellular cholesterol. Therefore, a failure of LDL uptake would prevent this repression, leading to uncontrolled synthesis of cholesterol by HMG-CoA reductase.