A 42-year-old man presents with recurrent nephrolithiasis and is found to have non-anion gap metabolic acidosis. Serum potassium is 2.8 mEq/L. Urine pH is 6.8 despite systemic acidosis. Urine anion gap is positive (+12). What is the MOST likely type of renal tubular acidosis?

• A Type II (proximal) RTA
• B Type IV RTA
• C Type III RTA
• D Type I (distal) RTA ✓

Explanation

Type I (distal) RTA is characterised by inability to acidify urine below pH 5.5 despite systemic acidosis (urine pH persistently >5.5 — here 6.8), hypokalaemia, positive urinary anion gap (impaired NH4+ excretion), nephrocalcinosis, and nephrolithiasis (calcium oxalate/phosphate stones due to hypercalciuria and high urine pH). Type II (proximal) RTA causes bicarbonate wasting with urine pH that can fall <5.5 during severe acidosis. Type IV RTA causes hyperkalaemia and is associated with aldosterone deficiency or resistance.

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

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