This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Anti-Neoplastic Agents, 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 Azathioprineo It is an analogue of mercaptopurine.o It selectively affects T cells -cellular immunity is predominantly affected,o It selectively affects dejferentiation and formation ofT cells,o Azathioprine is converted to active metabolite 6-mercaptopurine.o It can inhibit the synthesis of both DNA & RNA.o Azathioprine and 6-MP are metabolize by xanthine oxidase - dose should be reduced if allopurinol (a xanthine oxidase inhibitor) is used concurrently.o The most important action of azathioprine is prevention of renal and other graft rejection.