A 28-year-old man with recurrent nephrolithiasis has serum potassium 3.1 mEq/L, serum bicarbonate 14 mEq/L, serum chloride 115 mEq/L, urinary pH 6.8 (non-acidic despite systemic acidosis), and urine anion gap (UAG) is positive (+12). The MOST likely diagnosis is:
- A Type 2 (proximal) renal tubular acidosis
- B Type 4 renal tubular acidosis (hyperkalaemic)
- C Type 1 (distal) renal tubular acidosis ✓
- D Diarrhoea-induced metabolic acidosis
Correct answer: C. Type 1 (distal) renal tubular acidosis
Explanation
Type 1 (distal) RTA is characterised by: inability to acidify urine below pH 5.5 (urine pH ≥ 6.0 despite systemic acidosis), hypokalaemia, positive urine anion gap (UAG = Na+K−Cl > 0, indicating impaired NH4+ excretion), hyperchloraemic normal-anion-gap metabolic acidosis, and nephrocalcinosis/nephrolithiasis from alkaline urine and hypercalciuria. Type 2 RTA shows urine pH that can fall below 5.5 (the defect is in proximal bicarbonate reabsorption, not distal H+ secretion). Type 4 RTA has hyperkalaemia. Diarrhoea causes extrarenal HCO3 loss with negative UAG.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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