Which of the following chemotherapy drugs is classified as an antimetabolite?

5-Fluorouracil is classified as an antimetabolite because it disrupts the synthesis of nucleic acids, specifically by inhibiting thymidylate synthase, an enzyme crucial for DNA synthesis. By mimicking the structure of the normal nucleotides needed for DNA, it effectively interferes with the cellular division process, which is particularly impactful on rapidly proliferating cancer cells. Antimetabolites are designed to imitate the building blocks of DNA or RNA, impairing their production and leading to cell death.

In contrast, the other drugs mentioned do not fall under the antimetabolite category. Cyclophosphamide is an alkylating agent that works by directly damaging DNA, leading to cell death. Docetaxel is a taxane that disrupts microtubule function during cell division, while Etoposide is a topoisomerase inhibitor that interferes with the enzyme's function, preventing DNA unwinding and replication. Each of these agents has a different mechanism of action that does not involve the metabolic pathways of nucleotide synthesis, further highlighting why 5-Fluorouracil is correctly categorized as an antimetabolite.