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: Biochemistry
First product of purine metabolism 
Select and explain. 
A. Uric acid, B. Xanthine, C. P-alanine, D. CO2
B: Xanthine
Answer: B: Xanthine 
Explanation: Humans catabolize purines to uric acid. But, first purines are catabolized to xanthine, which is fuher catabolized to purine.