A 60-year-old diabetic man has eGFR 22 mL/min/1.73 m² and serum potassium 5.8 mEq/L. He is on losartan 50 mg/day. Arterial blood gas shows pH 7.30, HCO3 16 mEq/L, pCO2 32 mmHg. Urine anion gap is +12. What is the acid-base disorder and most likely cause of the hyperchloraemic metabolic acidosis?

• A Type 1 (distal) RTA
• B Type 2 (proximal) RTA
• C Lactic acidosis
• D Type 4 RTA (hyperkalaemic) due to hypoaldosteronism ✓

Explanation

Type 4 RTA (hyperchloraemic metabolic acidosis + hyperkalaemia) results from hypoaldosteronism or aldosterone resistance. In diabetic nephropathy, hyporeninism leads to hypoaldosteronism (type IV RTA); ACE inhibitors/ARBs (losartan) further suppress aldosterone, exacerbating hyperkalaemia and acidosis. Positive urine anion gap (+12) confirms impaired renal NH4+ excretion (rather than GI bicarbonate loss). Type 1 RTA features hypokalaemia and inability to acidify urine below pH 5.5. Type 2 RTA shows hypokalaemia and a urine pH that can be acidified.

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

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