The ROMK channel (Kir1.1, encoded by KCNJ1) in the thick ascending limb and cortical collecting duct is critical for K⁺ recycling in TAL and K⁺ secretion in CCD. Genetic loss of ROMK results in which condition and what is the paradoxical electrolyte finding?

• A Gitelman syndrome; hypomagnesemia and hypocalciuria
• B Liddle syndrome; hypertension with hypokalemia and metabolic alkalosis
• C Pseudohypoaldosteronism type 1; hyperkalemia with normal renal function
• D Bartter syndrome type 2; neonatal salt wasting with paradoxical hyperkalemia at birth then hypokalemia ✓

Explanation

ROMK mediates luminal K⁺ recycling in the TAL — essential for maintaining adequate lumenal K⁺ to support the Na-K-2Cl cotransporter (NKCC2). ROMK loss causes Bartter syndrome type 2. Paradoxically, neonates present with transient hyperkalemia because ROMK is also the principal secretory K⁺ channel in the cortical collecting duct; after birth, aldosterone-driven K⁺ secretion through ROMK is absent. Later, compensatory mechanisms (BK channels) develop and hypokalemia supervenes. This transient neonatal hyperkalemia distinguishes type 2 from other Bartter subtypes.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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