Which of the following is an example of an antimetabolite?

Methotrexate is an example of an antimetabolite because it interferes with the synthesis of nucleic acids by inhibiting the enzyme dihydrofolate reductase. This enzyme is crucial for the reductive metabolism of folates, which are necessary for the production of DNA and RNA. By mimicking folic acid, methotrexate disrupts normal cellular processes, particularly in rapidly dividing cells, such as cancer cells. This is why methotrexate is commonly used in chemotherapy for certain types of cancer as well as in the treatment of autoimmune diseases.

In contrast, the other drugs listed have different mechanisms of action. Cyclophosphamide is an alkylating agent that introduces alkyl groups into DNA, leading to cell death, particularly in cancer cells. Doxorubicin is an anthracycline antibiotic that intercalates DNA, disrupting replication and function and resulting in cell death. Vinblastine is a vinca alkaloid that inhibits microtubule formation, thereby blocking cell division. Each of these agents targets cancer cells through distinct mechanisms, distinguishing them from antimetabolites like methotrexate.