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 antiumor agents works by impairing de novo purine synthesis? 
Select and explain. 
A. Acyloguanosine, B. 5-Fluorouracil, C. Methotrexate (antifolate), D. Allopurinol
C: Methotrexate (antifolate)
Answer: C: Methotrexate (antifolate) 
Explanation: Ans. is 'c' i.e., Methotrexate (antifolate) o Methotrexate and permetrexed are dihydrofolate reductase (DHF Rase) inhibitors. o These drug also inhibit thymidylate synthase and de novo purine synthesis, which contribute to methotrexate toxicity.