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
Allopurinol prevents conversion of - 
Select and explain. 
A. Hypoxanthine to xanthine, B. Xanthine to hypoxanthine, C. Hypoxanthine to I.M.P, D. All
A: Hypoxanthine to xanthine
Answer: A: Hypoxanthine to xanthine 
Explanation: Ans. is 'a' i.e., Hypoxanthine to xanthine