Renal tubular acidosis type I (distal RTA) is characterised by inability to acidify urine below pH 5.5. The enzyme whose deficiency in the alpha-intercalated cells of the collecting duct directly causes this defect is:

• A Carbonic anhydrase II (CAII) in the cytoplasm of proximal tubular cells
• B H+-ATPase (vacuolar proton pump) in the luminal membrane of alpha-intercalated cells ✓
• C Na-K-ATPase in the basolateral membrane of collecting duct principal cells
• D Carbonic anhydrase IV (CAIV) on the luminal surface of proximal tubular cells

Explanation

Distal RTA type I results from defective proton secretion in the collecting duct. The luminal (apical) H+-ATPase (V-type proton pump) in alpha-intercalated cells is responsible for active secretion of H+ into the tubular lumen; its dysfunction prevents generation of a steep H+ gradient and acidification of urine below pH 5.5. Mutations in the B1 subunit of H+-ATPase cause autosomal recessive distal RTA with sensorineural hearing loss. CA II deficiency causes combined proximal + distal RTA (type III). CAIV deficiency causes proximal RTA (type II).

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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Written and medically reviewed by the StethoPrep medical team.