A 62-year-old woman with long-standing type 2 diabetes and mild retinopathy has serum potassium 5.7 mEq/L, bicarbonate 17 mEq/L, chloride 112 mEq/L, and an anion gap of 11 mEq/L. Arterial pH is 7.33. She takes no RAAS-blocking drugs. Urine pH is 5.0 despite acidaemia. The underlying defect is:

• A Impaired proximal tubular bicarbonate reabsorption
• B Defective distal tubular hydrogen ion secretion
• C Hyporeninaemic hypoaldosteronism ✓
• D Excess mineralocorticoid activity

Explanation

Type 4 RTA is the commonest RTA in adults and is typically seen in diabetic nephropathy as hyporeninaemic hypoaldosteronism. Reduced aldosterone impairs distal potassium and hydrogen secretion, producing hyperkalaemia with a non-anion gap acidosis. Unlike type 1 RTA, the urine can still acidify below pH 5.5 because some hydrogen secretion persists once hyperkalaemia is corrected. Proximal bicarbonate wasting defines type 2 RTA, which features hypokalaemia.

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

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Written and medically reviewed by the StethoPrep medical team.