This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunology and Rheumatology, Subject name: Medicine
A 74-year-old man with a previous episode of gout is given allopurinol. Which of the following is the most likely mechanism of action for this medication? 
Select and explain. 
A. increasing uric acid production, B. blocking excretion of uric acid by renal tubular mechanism, C. inhibiting xanthine oxidase, D. diminishing inflammation of acute gouty arthritis
C: inhibiting xanthine oxidase
Answer: C: inhibiting xanthine oxidase 
Explanation: Allopurinol effectively blocks uric acid production by inhibiting xanthine oxidase. Allopurinol is indicated in patients with a history of uric acid calculi of the urinary tract. In addition, it is often used in patients with malignancy (e.g., leukemia, lymphoma), particularly when chemotherapy or radiation therapy is being used.