This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General Pharmacology, Subject name: Pharmacology
6-Mercaptopurine (6-MP), frequently used in drug regimens for neoplastic disease, is metabolized by xanthine oxidase. Major dose reductions are advised in patients who are being treated with 
Select and explain. 
A. cimetidine, B. sulfinpyrazone, C. allopurinol, D. indomethacin
C: allopurinol
Answer: C: allopurinol 
Explanation: Allopurinol is a uricosuric drug used in chronic gout that prevents formation of uric acid from purines by acting as a suicide substrate of xanthine oxidase. The drug is commonly used in patients undergoing treatment of cancer to slow down formation of uric acid derived from purines released by the cytotoxic action of drugs or radiation. The metabolism of 6-mercaptopurine (6-MP), a substrate for xanthine oxidase, is also inhibited by allopurinol, necessitating a major dose reduction to avoid its toxic effects.