A patient with chronic diarrhoea presents with hypokalaemia (K⁺ 2.8 mEq/L), metabolic acidosis (HCO₃⁻ 14 mEq/L), and urine pH consistently 5.2. Urine anion gap is negative. What type of renal tubular acidosis (RTA) does this pattern suggest?
- A No RTA; this is consistent with GI bicarbonate loss (non-anion gap metabolic acidosis with intact renal acidification) ✓
- B Type 1 (distal) RTA
- C Type 2 (proximal) RTA
- D Type 4 (hyperkalaemic) RTA
Correct answer: A. No RTA; this is consistent with GI bicarbonate loss (non-anion gap metabolic acidosis with intact renal acidification)
Explanation
In hyperchloraemic non-anion gap metabolic acidosis, a negative urine anion gap (UAG = [Na⁺+K⁺]−Cl⁻ urine) indicates increased urinary ammonium excretion (NH₄⁺), reflecting intact renal acidification — the kidney is appropriately responding to the systemic acidosis. Combined with urine pH <5.5, this points to an extra-renal (GI) source of bicarbonate loss such as diarrhoea. Type 1 (distal) RTA shows urine pH >5.5 and positive UAG. Type 4 RTA presents with hyperkalaemia. Type 2 (proximal) RTA has a variable urine pH.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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