Bartter Type I SLC12A1 NKCC2 Defect — ESENeph MCQ

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HardTubular DisordersBartter Type I SLC12A1 NKCC2 DefectESENeph

A 27-year-old man with Bartter syndrome type I (SLC12A1 mutation) presents with muscle cramps and fatigue. Bloods: K+ 2.5 mmol/L, Mg2+ 0.6 mmol/L, bicarbonate 32 mmol/L, renin elevated, aldosterone elevated. BP 100/65 mmHg. Urine calcium is elevated. What is the channel defect?

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Correct answer: D — Furosemide-sensitive NKCC2 in the thick ascending limb

SLC12A1 encodes NKCC2 (Na-K-2Cl cotransporter), the target of loop diuretics, located on the luminal membrane of the thick ascending limb (TAL). Loss-of-function mutations cause Bartter syndrome type I — the most severe neonatal form with polyhydramnios, prematurity, salt wasting, hypokalaemic alkalosis, and hypercalciuria (with risk of nephrocalcinosis). The clinical phenotype mimics chronic furosemide use. In contrast, Bartter type II involves ROMK (KCNJ1), type III involves ClC-Kb (CLCNKB), and Bartter type IV involves barttin (BSND). Gitelman syndrome involves NCC (SLC12A3) in the DCT and characteristically has HYPOcalciuria. Treatment includes potassium and magnesium supplementation, indomethacin, and spironolactone.

Reference: Seyberth & Schlingmann 2011 – Bartter Classification; JRCPTB 2022 – Nephrology Curriculum