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
A patient receiving allopurinol requires dose reduction of: 
Select and explain. 
A. 6–Mercaptopurine, B. Cyclophosphamide, C. 6–Thioguanine, D. Cimetidine
A: 6–Mercaptopurine
Answer: A: 6–Mercaptopurine 
Explanation: 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.
6-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.