A 25-year-old woman has recurrent nephrolithiasis. Urine pH is consistently 6.5-7.0. Serum bicarbonate is 16 mEq/L, potassium 2.8 mEq/L, and chloride 112 mEq/L. Urine anion gap is positive. Which type of renal tubular acidosis (RTA) is MOST likely?
- A Type 1 RTA (distal RTA) ✓
- B Type 2 RTA (proximal RTA)
- C Type 4 RTA (hyperkalemic RTA)
- D Normal anion gap acidosis from diarrhoea
Correct answer: A. Type 1 RTA (distal RTA)
Explanation
Type 1 (distal) RTA is characterised by inability to acidify urine below pH 5.5, leading to urine pH >5.5 in the setting of systemic acidosis, normal anion gap hyperchloraemic metabolic acidosis, hypokalaemia (due to secondary hyperaldosteronism), and positive urine anion gap. Nephrolithiasis (calcium phosphate stones) and nephrocalcinosis are hallmarks. Type 2 RTA shows urine pH that can fall below 5.5 with severe acidosis. Type 4 RTA has hyperkalaemia. Diarrhoea causes metabolic acidosis with negative urine anion gap.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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