Methotrexate is best described as which type of drug and what is its core mechanism?

• A. A nonsteroidal anti-inflammatory drug.
• B. A monoclonal antibody.
• C. A chemotherapy agent that acts as a folate analogue inhibiting dihydrofolate reductase.
• D. A chemotherapy agent that acts as a folate analogue and competitively inhibits dihydrofolate reductase, impairing DNA synthesis.

Answer: (D)

Methotrexate acts as a chemotherapy agent that mimics folate and competitively inhibits dihydrofolate reductase. By blocking DHFR, it prevents conversion of dihydrofolate to tetrahydrofolate, reducing the availability of cofactors needed to synthesize thymidylate and purines. This cripples DNA synthesis, leading to impaired replication in rapidly dividing cells. The competitive binding to DHFR and the resulting disruption of DNA synthesis is the key mechanism, which is why this option best describes the drug. It is not an NSAID or a monoclonal antibody.