This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cytotoxic Anticancer Drugs, Subject name: Pharmacology
Which of the following is a dihydrofolate reductase inhibitor? 
Select and explain. 
A. Alcohol, B. Methotrexate, C. Cimetidine, D. Erythromycin
B: Methotrexate
Answer: B: Methotrexate 
Explanation: Methotrexate, pemetrexed and pralatrexate are the inhibitors of dihydrofolate reductase (DHFRase). These drugs also inhibit Thymidylate Synthase (TS) and the enzymes involved in early purine synthesis. Methotrexate is the drug of choice for the treatment of choriocarcinoma. Methotrexate is also indicated in the management of rheumatoid ahritis, psoriasis and ectopic pregnancy. Adverse effects of methotrexate are bone marrow suppression and mucositis.