A patient with gout is started on allopurinol. Which statement correctly describes how this drug lowers serum urate?

• A Allopurinol blocks renal tubular reabsorption of uric acid
• B Allopurinol irreversibly degrades xanthine oxidase protein
• C Allopurinol competitively inhibits uricase in the liver
• D Allopurinol is a competitive inhibitor of xanthine oxidase, while its metabolite oxypurinol produces prolonged non-competitive inhibition ✓

Explanation

Allopurinol is a structural analog of hypoxanthine and competes with it for xanthine oxidase. During the reaction the enzyme converts it to oxypurinol (alloxanthine), which binds the molybdenum center of xanthine oxidase tightly and acts as a non-competitive, long-lasting inhibitor, explaining the sustained fall in urate after dosing. Uricase is absent in humans, so option C is physiologically impossible. Option A describes uricosuric drugs such as probenecid and lesinurad, a completely different mechanism.

Reference: Katzung Basic and Clinical Pharmacology, 16th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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