A hyperchloraemic normal anion gap metabolic acidosis with urinary anion gap (UAG) = +10 mEq/L suggests which diagnosis?

• A Diarrhoea (GI bicarbonate loss)
• B Proximal renal tubular acidosis (Type 2 RTA)
• C Distal renal tubular acidosis (Type 1 RTA) ✓
• D Lactic acidosis

Explanation

The urinary anion gap (Na + K − Cl) reflects unmeasured urinary ammonium (NH4+). A positive UAG indicates impaired NH4+ excretion — characteristic of distal (type 1) RTA where the collecting duct cannot acidify urine. In diarrhoea (GI loss), the kidney appropriately increases NH4+ excretion, giving a negative UAG (−10 to −30). In type 2 RTA, UAG may be negative (NH4+ excretion is intact). Lactic acidosis has elevated anion gap, not normal. Type 1 RTA also shows urine pH >5.5 in severe acidosis.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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