Allopurinol reduces uric acid production by inhibiting which enzyme?

• A. Adenosine deaminase
• B. Purine phosphoribosyltransferase
• C. Lactate dehydrogenase
• D. Xanthine oxidase

Answer: (D)

Allopurinol works by inhibiting xanthine oxidase, the enzyme that catalyzes the final steps of purine breakdown converting hypoxanthine to xanthine and xanthine to uric acid. By blocking this enzyme (and its active metabolite oxypurinol), uric acid production drops and the pathway shifts toward soluble precursors like hypoxanthine and xanthine, which are more easily excreted. The other enzymes listed aren’t involved in the final uric acid formation step: adenosine deaminase acts earlier in purine catabolism, purine phosphoribosyltransferase is part of the purine salvage pathway, and lactate dehydrogenase is part of glycolysis.