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
Active metabolite of azathioprine ? 
Select and explain. 
A. 6-thioguanine, B. 6-thiouracil, C. 6-mercaptopurine, D. 6-mercaptoguanine
C: 6-mercaptopurine
Answer: C: 6-mercaptopurine 
Explanation: Ans. is 'c' i.e., 6-mercaptopurine Azathioprine It is an analogue of mercaptopurine. It selectively affects T cells - cellular immunity is predominantly affected. It selectively affects defferentiation and formation of T cells. Azathioprine is conveed to active metabolite 6-mercaptopurine. It can inhibit the synthesis of both DNA & RNA. Azathioprine and 6-MP are metabolize by xanthine oxidase -dose should be reduced if allopurinol (a xanthine oxidase inhibitor) is used concurrently. The most impoant action of azathioprine is prevention of renal and other graft rejection.