A 20-year-old woman presents with muscle cramps and one episode of carpopedal spasm. Labs: K 2.7 mEq/L, Mg 1.1 mg/dL, HCO3 32 mEq/L, pH 7.50, Cl 96 mEq/L. Blood pressure is 108/70 mmHg. Spot urine calcium to creatinine ratio is low. Plasma renin and aldosterone are both elevated. Which additional finding best separates this disorder from Bartter syndrome?
- A Hypercalciuria with nephrocalcinosis
- B Low renin with high aldosterone and hypertension
- C Hypomagnesaemia with hypocalciuria due to defective TRPM6 channel function in the distal convoluted tubule ✓
- D Metabolic acidosis with alkaline urine
Correct answer: C. Hypomagnesaemia with hypocalciuria due to defective TRPM6 channel function in the distal convoluted tubule
Explanation
Gitelman syndrome results from loss-of-function mutations in the thiazide-sensitive sodium chloride cotransporter in the distal convoluted tubule, producing a phenotype like chronic thiazide use: hypokalaemic metabolic alkalosis with hypomagnesaemia and hypocalciuria. Bartter syndrome mimics loop diuretic effect and features normal or high urinary calcium, often with nephrocalcinosis. Both have elevated renin and aldosterone with normal or low blood pressure, so the magnesium level and urinary calcium separate them.
Reference: Williams Textbook of Endocrinology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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