Which of the following medications is an antimetabolite?

Methotrexate Sodium is classified as an antimetabolite because it interferes with the metabolic processes of cells, particularly in the synthesis of nucleic acids. This action is primarily due to its ability to inhibit the enzyme dihydrofolate reductase, which is crucial for the production of tetrahydrofolate, an essential cofactor involved in DNA and RNA synthesis. By disrupting this pathway, methotrexate can effectively inhibit the growth of rapidly dividing cells, such as cancer cells, which is why it is often utilized in chemotherapy regimens and the treatment of autoimmune diseases.

In contrast, other medications listed serve different purposes. Olanzapine is an atypical antipsychotic used primarily for treating schizophrenia and bipolar disorder. Dipyridamole is a vasodilator that is typically used to prevent blood clots, often in conjunction with other anticoagulants. Nitrofurantoin is an antibiotic that is utilized for treating urinary tract infections, operating through mechanisms that target bacterial cell wall synthesis and function. Each of these alternatives acts via different mechanisms that do not involve the inhibition of metabolic pathways relevant to nucleic acid synthesis, which is the defining characteristic of antimetabolites like methotrexate.