A 20-year-old woman with lifelong normotensive hypokalaemic metabolic alkalosis has urine calcium-to-creatinine ratio of 0.08 (low), serum magnesium 1.2 mg/dL, and normal renin-independent aldosterone levels after saline challenge. The most likely defect involves:

• A Thiazide-sensitive NCC cotransporter in the distal convoluted tubule ✓
• B NKCC2 cotransporter in the thick ascending limb
• C Epithelial sodium channel ENaC in the collecting duct
• D ROMK channel mutation causing potassium wasting

Explanation

Gitelman syndrome results from loss-of-function mutations of the thiazide-sensitive NaCl cotransporter (NCC) in the distal convoluted tubule. Its signature features are hypokalaemic metabolic alkalosis with hypomagnesaemia and HYPOcalciuria. Bartter syndrome, caused by defects such as NKCC2 loss, shares the alkalosis but shows HYPERcalciuria and usually normal magnesium. ENaC activation causes Liddle syndrome with hypertension, and ROMK mutation is a form of Bartter syndrome type II.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.