PLAB 1 Diagnosis & Investigation Interpretation — Questions and Answers
Question 1: A 55-year-old man has the following arterial blood gas results: pH 7.28, PaCO2 3.2 kPa, PaO2 12.5 kPa, HCO3 14 mmol/L. What is the most likely acid-base disturbance?
- Respiratory acidosis
- Metabolic acidosis with respiratory compensation (Correct answer)
- Respiratory alkalosis
- Mixed metabolic and respiratory acidosis
Correct answer: Metabolic acidosis with respiratory compensation
pH 7.28 = acidosis. Low HCO3 (14) = metabolic acidosis. Low PaCO2 (3.2) = compensatory hyperventilation (blowing off CO2 to raise pH). This is metabolic acidosis with appropriate respiratory compensation. The anion gap should be calculated to determine the cause.
Question 2: A 30-year-old woman has blood test results showing: Hb 88 g/L, MCV 70 fL, serum ferritin 6 µg/L, transferrin saturation 8%. What is the most likely diagnosis?
- Anaemia of chronic disease
- Iron deficiency anaemia (Correct answer)
- Thalassaemia trait
- Folate deficiency anaemia
Correct answer: Iron deficiency anaemia
Iron deficiency anaemia is characterised by low Hb, microcytic anaemia (low MCV <80 fL), low ferritin (the most sensitive indicator of iron stores), and low transferrin saturation. Ferritin of 6 µg/L confirms depleted iron stores. Anaemia of chronic disease has normal/raised ferritin.
Question 3: A 68-year-old man has a serum sodium of 122 mmol/L. He has gained weight, has peripheral oedema, and his urine sodium is 10 mmol/L. What is the most likely cause?
- SIADH (syndrome of inappropriate ADH)
- Cardiac failure or nephrotic syndrome causing dilutional hyponatraemia (Correct answer)
- Addison's disease
- Diuretic therapy
Correct answer: Cardiac failure or nephrotic syndrome causing dilutional hyponatraemia
Dilutional hyponatraemia in a hypervolaemic patient (oedema, weight gain) with low urine sodium (the kidney is avidly retaining sodium) points to cardiac failure, cirrhosis, or nephrotic syndrome. In SIADH, patients are euvolaemic with urine sodium >20 mmol/L. The kidneys are working normally but the total body water is excessive.
Question 4: A chest X-ray shows a tracheal deviation to the right, loss of left lung volume, and a raised left hemidiaphragm. What is the most likely diagnosis?
- Left pneumothorax
- Left sided pleural effusion
- Left sided lung collapse (atelectasis) (Correct answer)
- Right pneumothorax
Correct answer: Left sided lung collapse (atelectasis)
Collapse (atelectasis) causes loss of volume on the affected side: trachea and mediastinum deviate towards the collapse, the hemidiaphragm rises, and lung volume is reduced. In tension pneumothorax or large effusion, the mediastinum deviates away from the affected side.
Question 5: A patient's ECG shows: heart rate 150 bpm, narrow complex, no visible P waves, irregularly irregular rhythm. What is the most likely diagnosis?
- Atrial flutter with 2:1 block
- Atrial fibrillation (Correct answer)
- SVT (AVNRT)
- Sinus tachycardia
Correct answer: Atrial fibrillation
Atrial fibrillation (AF) is characterised by: (1) irregularly irregular rhythm — the most important feature; (2) absence of distinct P waves (replaced by fibrillatory baseline); (3) narrow complexes (unless aberrant conduction). SVT is regular; atrial flutter is typically regular at 150 bpm with flutter waves.
Question 6: A 25-year-old woman has TFTs showing: TSH <0.01 mIU/L, free T4 35 pmol/L, free T3 12 pmol/L. What do these results indicate?
- Primary hypothyroidism
- Hyperthyroidism (primary) (Correct answer)
- Secondary hypothyroidism
- Sick euthyroid syndrome
Correct answer: Hyperthyroidism (primary)
A suppressed TSH (<0.1) with elevated free T4 and T3 indicates primary hyperthyroidism. The high thyroid hormones are suppressing TSH via negative feedback on the pituitary. In secondary hypothyroidism (pituitary failure), TSH is low but T4 is also low.
