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 of the following drugs is topoisomerase I inhibitor? 
Select and explain. 
A. Doxorubicin, B. Irinotecan, C. Etoposide, D. Vincristine
B: Irinotecan
Answer: B: Irinotecan 
Explanation: Camptothecin derivatives like topotecan and irinotecan act by inhibiting topoisomerase-I whereas Epipodophyllotoxins (etoposide and teniposide) and anthracyclines (doxorubicin, daunorubicin etc.) act by inhibiting topoisomerase-II.