A 58-year-old man with diabetic nephropathy (eGFR 30 mL/min) on maximum ACE inhibitor presents with serum K+ 6.2 mEq/L. He has no ECG changes. Plasma renin is low and aldosterone is inappropriately low-normal. Urine pH is 5.0. Which diagnosis best explains the hyperkalemia?

• A Type 1 distal renal tubular acidosis
• B Type 4 (hyporeninemic hypoaldosteronism) RTA ✓
• C Type 2 proximal renal tubular acidosis
• D Primary adrenal insufficiency

Explanation

Type 4 RTA is caused by hyporeninemic hypoaldosteronism, common in diabetic nephropathy. It causes hyperkalemia with normal anion gap metabolic acidosis and appropriately acidic urine (pH <5.5). Type 1 RTA causes hypokalemia with high urine pH. Type 2 RTA causes hypokalemia with proximal dysfunction. Primary adrenal insufficiency causes high renin with low aldosterone.

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

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