MRCP Part 1 Clinical Sciences — Questions and Answers
Question 1: A 45-year-old man presents with muscle weakness and hypokalaemia. His arterial blood gas shows metabolic alkalosis. Which hormone, if produced in excess, is most likely responsible?
- Aldosterone (Correct answer)
- Cortisol
- Adrenaline
- Thyroxine
Correct answer: Aldosterone
Excess aldosterone (Conn's syndrome) causes sodium retention and potassium excretion, leading to hypokalaemia and metabolic alkalosis. This is a classic presentation of primary hyperaldosteronism.
Question 2: Which immunoglobulin class is found in highest concentration in normal human serum?
- IgG (Correct answer)
- IgA
- IgM
- IgE
Correct answer: IgG
IgG constitutes approximately 75-80% of serum immunoglobulins and is the most abundant antibody class in blood. It crosses the placenta and provides passive immunity to neonates.
Question 3: A patient with chronic kidney disease has a corrected calcium of 1.9 mmol/L and phosphate of 2.1 mmol/L. Which is the most likely underlying mechanism?
- Reduced renal 1-alpha-hydroxylase activity (Correct answer)
- Increased PTH secretion
- Vitamin D toxicity
- Primary hyperparathyroidism
Correct answer: Reduced renal 1-alpha-hydroxylase activity
In CKD, the kidneys cannot convert 25-hydroxyvitamin D to its active form (1,25-dihydroxyvitamin D) due to reduced 1-alpha-hydroxylase activity. This leads to decreased calcium absorption, hypocalcaemia, and hyperphosphataemia from reduced renal phosphate excretion.
Question 4: Which type of hypersensitivity reaction is responsible for Goodpasture's syndrome?
- Type II (cytotoxic) (Correct answer)
- Type I (immediate)
- Type III (immune complex)
- Type IV (delayed)
Correct answer: Type II (cytotoxic)
Goodpasture's syndrome is a type II hypersensitivity reaction where autoantibodies target the alpha-3 chain of type IV collagen in glomerular and alveolar basement membranes, causing glomerulonephritis and pulmonary haemorrhage.
Question 5: A 30-year-old woman has a serum sodium of 118 mmol/L, serum osmolality of 260 mOsm/kg, and urine osmolality of 550 mOsm/kg. She is clinically euvolaemic. What is the most likely diagnosis?
- Syndrome of inappropriate ADH secretion (SIADH) (Correct answer)
- Addison's disease
- Psychogenic polydipsia
- Nephrotic syndrome
Correct answer: Syndrome of inappropriate ADH secretion (SIADH)
SIADH presents with hyponatraemia, low serum osmolality, and inappropriately concentrated urine (>100 mOsm/kg) in a euvolaemic patient. Psychogenic polydipsia would show dilute urine, and Addison's would typically show hypovolaemia.
Question 6: Which enzyme deficiency is responsible for acute intermittent porphyria?
- Porphobilinogen deaminase (Correct answer)
- ALA synthase
- Ferrochelatase
- Uroporphyrinogen decarboxylase
Correct answer: Porphobilinogen deaminase
Acute intermittent porphyria is caused by deficiency of porphobilinogen (PBG) deaminase (also called hydroxymethylbilane synthase), the third enzyme in the haem biosynthesis pathway. This leads to accumulation of ALA and PBG, causing neurovisceral attacks.
MRCP Part 1 Clinical Sciences
A 45-year-old man presents with muscle weakness and hypokalaemia.
His arterial blood gas shows metabolic alkalosis.
Which hormone, if produced in excess, is most likely responsible?