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
Methotrexate mechanism of action: 
Select and explain. 
A. Inhibit dihydrofolate reductase, B. Stimulate dihydrofolate reductase, C. Inhibit tetrahydrofolate reductase, D. Stimulate tetrahydrofolate reductase
A: Inhibit dihydrofolate reductase
Answer: A: Inhibit dihydrofolate reductase 
Explanation: Ans. (a) Inhibit dihydrofolate ReductaseRef. KD Tripathi 6th Ed. / 823METHOTREXATE (MTX)DHFADHFRase---------------THFA* Methotrexate is a highly efficacious antineoplastic drug which inhibits dihydrofolate reductase (DHFRase) enzyme- blocking the conversion of dihydrofolic acid (DHFA) to tetrahydrofolic acid (THFA).* This conversion is essential for formation of an essential coenzyme required for one carbon transfer reactions in denovo purine synthesis and amino acid interconversions.* Methotrexate has cell cycle specific action kills cells in S phase; primarily inhibits DNA synthesis.Also Know*Drug of choice for Mtx toxicity: FOLINIC ACID.