A 24-year-old woman presents with recurrent muscle cramps and weakness. BP is 104/66 mmHg. Labs: potassium 2.6 mEq/L, magnesium 1.2 mg/dL, bicarbonate 30 mEq/L, renin elevated, aldosterone elevated. Spot urine calcium-to-creatinine ratio is very low. The genetic defect lies in:

• A NKCC2 cotransporter in the thick ascending limb
• B Aquaporin-2 water channel in the collecting duct
• C Epithelial sodium channel ENaC
• D Thiazide-sensitive NaCl cotransporter in the distal convoluted tubule ✓

Explanation

Gitelman syndrome is caused by loss-of-function mutations in SLC12A3 encoding the thiazide-sensitive sodium-chloride cotransporter of the distal convoluted tubule. It mimics chronic thiazide use: hypokalaemic metabolic alkalosis with hypomagnesaemia and strikingly low urinary calcium. Bartter syndrome, due to NKCC2 defects in the thick ascending limb, instead shows normal or high urinary calcium and usually presents earlier with polyhydramnios or failure to thrive.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.