A 55-year-old man is found to have renal tubular acidosis (RTA). His urine pH is persistently above 5.5 despite systemic acidosis (serum pH 7.28). Urine anion gap is positive (+8 mEq/L). Which type of RTA does this represent, and where is the defect?
- A Type 1 (distal) RTA — failure of alpha-intercalated cells to secrete H+ via H+-ATPase ✓
- B Type 2 (proximal) RTA — failure of proximal tubule to reabsorb HCO3
- C Type 4 RTA — hypoaldosteronism reducing K+ secretion and impairing NH4+ excretion
- D Type 3 RTA — combined proximal and distal defect
Correct answer: A. Type 1 (distal) RTA — failure of alpha-intercalated cells to secrete H+ via H+-ATPase
Explanation
Type 1 (distal) RTA is characterized by inability to acidify urine below pH 5.5 despite systemic acidosis, because alpha-intercalated cells in the collecting duct cannot secrete H+ against a steep gradient. The urine anion gap (Na + K − Cl) is positive because NH4+ excretion is low (positive UAG indicates absent ammonium). In Type 2 RTA, the urine can acidify when plasma HCO3 falls below the reabsorptive threshold. Type 4 RTA presents with hyperkalemia and only modest acidosis. Distal RTA is associated with nephrocalcinosis, nephrolithiasis, and hypokalemia.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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