A 28-year-old man with recurrent nephrolithiasis has serum potassium 3.1 mEq/L, bicarbonate 14 mEq/L, urine pH 6.8 (inappropriately alkaline), urine anion gap +12. Urine calcium is elevated. Which type of renal tubular acidosis does he have, and what is the underlying defect?

• A Proximal (Type 2) RTA; failure of bicarbonate reabsorption in proximal tubule
• B Type 4 RTA; aldosterone deficiency causing hyperkalaemia and acidosis
• C Type 3 RTA; combined proximal and distal defect due to carbonic anhydrase II deficiency
• D Distal (Type 1) RTA; inability to acidify urine due to defective H+ secretion in collecting duct ✓

Explanation

Distal (Type 1) RTA presents with hypokalemia, normal anion gap metabolic acidosis, inappropriately alkaline urine (pH consistently >5.5) despite systemic acidosis, and hypercalciuria with nephrolithiasis and nephrocalcinosis. The defect is failure of H+ secretion (H+/ATPase) in the alpha-intercalated cells of the collecting duct. Positive urine anion gap confirms impaired NH4+ excretion. Type 2 RTA causes bicarbonate wasting with urine pH <5.5 during acidaemia. Type 4 RTA causes hyperkalaemia. Type 3 is a rare combined defect.

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

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Written and medically reviewed by the StethoPrep medical team.