Methotrexate is an antineoplastic agent that destroys thymine.

• A. Methotrexate
• B. Busulfan
• C. Vincristine
• D. Doxorubicin

Answer: (A)

Methotrexate works by blocking dihydrofolate reductase, which reduces the available tetrahydrofolate needed for one-carbon transfer reactions. This shuts down the synthesis of thymidylate (dTMP), a building block of DNA. Without enough dTMP, DNA replication in rapidly dividing cells halts, leading to cell death. It doesn’t destroy thymine itself; it prevents the production of thymidine necessary for DNA synthesis. The other drugs act by different ways: busulfan cross-links DNA, vincristine disrupts microtubule formation, and doxorubicin intercalates DNA and generates reactive radicals. So the mechanism that best fits the statement is methotrexate’s inhibition of thymidylate synthesis via folate antagonism.