A 35-year-old woman presents with recurrent renal stones (calcium oxalate) and is found to have hyperchloraemic non-anion-gap metabolic acidosis, serum potassium 2.8 mmol/L, and urine pH 6.5 despite serum pH 7.28. 24-hour urine citrate is low. This presentation is MOST consistent with:
- A Type 2 (proximal) RTA — bicarbonate wasting in proximal tubule
- B Type 4 (hyperkalemic) RTA — aldosterone deficiency/resistance
- C Diarrhoea-induced metabolic acidosis
- D Type 1 (distal) RTA — failure to acidify urine below pH 5.5 ✓
Explanation
Type 1 (distal) RTA is characterised by inability to acidify urine below pH 5.5, resulting in a persistently alkaline urine (pH >5.5) even in the presence of systemic acidosis. The combination of hyperchloraemic non-anion-gap metabolic acidosis, hypokalaemia (collecting duct H+/K+-ATPase dysfunction), nephrolithiasis (calcium phosphate or oxalate stones — due to alkaline urine and hypercalciuria), nephrocalcinosis, and low urinary citrate (which normally inhibits stone formation) is pathognomonic of distal RTA. Type 2 RTA also causes hypokalaemia and NAGMA but urine pH falls <5.5 when plasma bicarbonate drops below reabsorptive threshold. Type 4 causes hyperkalaemia.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.