<prompt> Methotrexate mechanism of action:Inhibit dihydrofolate reductaseStimulate dihydrofolate reductaseInhibit tetrahydrofolate reductaseStimulate tetrahydrofolate reductase  <response>: Ans. a Inhibit dihydrofolate ReductaseRef. KD Tripathi 6th Ed. 823METHOTREXATE MTXDHFADHFRase---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 KnowDrug of choice for Mtx toxicity: FOLINIC ACID. 