A 64-year-old man with long-standing type 2 diabetes and diabetic nephropathy (eGFR 30 mL/min) has serum potassium 5.9 mEq/L, bicarbonate 18 mEq/L, chloride 110 mEq/L, and anion gap 12 mEq/L. Urine pH is 5.0 despite systemic acidaemia, and urine anion gap is strongly positive. He is not on any RAAS-blocking agent. Which disorder best explains these findings?

• A Type 1 distal RTA with impaired hydrogen ion secretion
• B Type 4 RTA from hyporeninaemic hypoaldosteronism ✓
• C Type 2 proximal RTA with bicarbonate wasting
• D Chronic diarrhoea with secondary hyperaldosteronism

Explanation

Hyperkalaemic normal anion gap metabolic acidosis in a diabetic with reduced GFR defines type 4 RTA, caused by hyporeninaemic hypoaldosteronism impairing ammoniagenesis and potassium secretion. The acidic urine pH of 5.0 is characteristic, because the tubule can still acidify once enough sodium reaches the distal nephron; the defect lies in ammonium excretion, shown by the positive urine anion gap. Type 1 RTA would show urine pH persistently above 5.5 with hypokalaemia.

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

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