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
Which one of the following drugs is `Topoisomerase 1 inhibitor ? 
Select and explain. 
A. Doxorubicin, B. Irinotecan, C. Etoposide, D. Vincristine
B: Irinotecan
Answer: B: Irinotecan 
Explanation: Ans. is 'b' i.e., Irinotecan Topoisomerase I inhibitors --> camptothecans (Irinotecan, tropotecan). Topoisomerase II inhibitors Anthracyclines (doxorubicin, daunorubicin) and Epipodophyllotoxin (Etoposid, teniposide).