Carbonic anhydrase II deficiency causes a rare syndrome of renal tubular acidosis, osteopetrosis, and cerebral calcification. The enzyme normally accelerates which reaction that is essential for urinary H+ secretion?

• A NH3 + H+ → NH4+, providing a urinary buffer in the collecting duct
• B H2PO4− → HPO42− + H+, generating titratable acid in the proximal tubule
• C Glutamine → glutamate + NH3, providing ammonium for buffering
• D CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3−, facilitating H+ secretion into the tubular lumen and HCO3− reabsorption into blood ✓

Explanation

Carbonic anhydrase II (CA-II) in proximal tubule cells, thick ascending limb, and intercalated cells catalyses the reversible hydration of CO2 to carbonic acid (H2CO3), which rapidly dissociates to H+ and HCO3−. In the proximal tubule, luminal CA-IV and intracellular CA-II work in concert: secreted H+ reacts with filtered HCO3− in the lumen forming H2CO3, which CA-IV dehydrates to CO2 + H2O; CO2 diffuses into the cell where CA-II rehydrates it to H+ (for secretion by NHE3) and HCO3− (for basolateral transport). In CA-II deficiency, this process fails, causing mixed type I and II RTA. The NH3/NH4+ and titratable acid pathways are independent of CA-II.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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