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 : a) Hypoxanthine to xanthine b) Xanthine to hypoxanthine c) Hypoxanthine to I.M.P. d) Xanthine to uric acid 
Select and explain. 
A. ab, B. cd, C. ad, D. bd
C: ad
Answer: C: ad 
Explanation: Xanthine oxidase catalyses the conversion of hypoxanthine to xanthine as well as xanthine to uric acid. This enzyme is inhibited by allopurinol.