This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
[{'content': 'A patient receiving allopurinol requires dose reduction of: \nSelect and explain. \nA. 6–Mercaptopurine, B. Cyclophosphamide, C. 6–Thioguanine, D. Cimetidine', 'role': 'user'}, {'content': "Answer: A \nExplanation: Allopurinol is an inhibitor of uric acid synthesis. It acts by inhibiting the enzyme 'xanthine oxidase. This drug is commonly prescribed to a patient on anticancer therapy because destruction of cells results in increased formation of uric acid.\n6-Mercaptopurine is also metabolized by xanthine oxidase. If allopurinol is administered to a patient receiving 6-MP, toxicity may occur due to increased plasma concentration of 6-MP. Therefore if both are given concurrently, dose of 6-MP should be decreased to 25% of the normal.", 'role': 'assistant'}]