A boy presents with failure to thrive, polyuria, and hypokalemic metabolic alkalosis with elevated renin and aldosterone. Workup shows hypercalciuria and normal serum magnesium. His sister had a similar illness but with hypomagnesemia, hypocalciuria, and onset in adolescence. These disorders result from defects in salt reabsorption in which two nephron segments respectively?

• A Proximal tubule in the brother, cortical collecting duct in the sister
• B Thick ascending limb in the brother, distal convoluted tubule in the sister ✓
• C Distal convoluted tubule in the brother, thick ascending limb in the sister
• D Thin descending limb in the brother, medullary collecting duct in the sister

Explanation

Bartter syndrome mimics a loop diuretic: defective NKCC2 or associated channels in the thick ascending limb cause salt wasting, hyperreninemic hyperaldosteronism, hypokalemic alkalosis, and impaired calcium reabsorption leading to hypercalciuria. Gitelman syndrome mimics a thiazide: defective NCCT in the distal convoluted tubule produces similar electrolyte abnormalities but with hypocalciuria and hypomagnesemia. Segment-specific physiology predicts the calcium phenotype.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.