A 35-year-old woman has recurrent kidney stones. Urine pH is persistently 6.5–7.0 despite hydration. Serum potassium is 3.0 mEq/L, HCO3 is 16 mEq/L. Urine anion gap is positive (+12). Urine pH fails to drop below 5.5 with ammonium chloride loading. She is non-acidotic at presentation. Which RTA type is this?
- A Type 1 (distal) RTA ✓
- B Type 2 (proximal) RTA
- C Type 4 (hyperkalaemic) RTA
- D Type 3 RTA
Correct answer: A. Type 1 (distal) RTA
Explanation
Distal (Type 1) RTA is characterised by inability to acidify urine below pH 5.5 even with acid loading, hypokalaemia (due to K-H exchange in collecting duct), positive urine anion gap (reflecting low NH4+ excretion), hyperchloraemic normal anion gap metabolic acidosis, and nephrocalcinosis/recurrent nephrolithiasis due to hypercalciuria and low urinary citrate. Type 2 (proximal) RTA shows urine pH <5.5 during established acidosis. Type 4 RTA shows hyperkalaemia. The persistent alkaline urine and nephrolithiasis are hallmarks of distal RTA.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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