In renal tubular acidosis (RTA), a patient has hyperchloraemic non-anion gap metabolic acidosis with serum K+ 5.8 mEq/L and urine pH 5.8. Urine anion gap is positive. This pattern is MOST consistent with:
- A Type 4 RTA (hyperkalaemic distal RTA / hypoaldosteronism) ✓
- B Type 1 RTA (distal classical, hypokalaemic)
- C Type 2 RTA (proximal bicarbonate wasting)
- D Diarrhoea-induced metabolic acidosis
Correct answer: A. Type 4 RTA (hyperkalaemic distal RTA / hypoaldosteronism)
Explanation
Type 4 RTA presents with hyperchloraemic non-anion gap metabolic acidosis and HYPERKALEMIA — due to aldosterone deficiency or resistance (hyporeninaemic hypoaldosteronism, seen in diabetic nephropathy and obstructive uropathy). Urine pH <5.5 confirms intact distal acidification ability (unlike Type 1 where urine pH >5.5). Positive urine anion gap confirms reduced ammonium excretion. Type 1 presents with hypokalaemia and urine pH >5.5. Type 2 presents with hypokalaemia and urine pH that falls below 5.5 with acid loading.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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