**Core Concept**
Antifolate drugs are a class of antineoplastic agents that inhibit the synthesis of DNA and RNA by targeting the folate pathway. This is crucial in cancer treatment, as rapidly dividing cancer cells rely heavily on folate metabolism.

**Why the Correct Answer is Right**
The correct antifolate drug is Methotrexate, which acts by inhibiting dihydrofolate reductase (DHFR). DHFR is an enzyme responsible for converting dihydrofolate to tetrahydrofolate, a crucial cofactor in the synthesis of thymidine, a nucleotide required for DNA replication. By inhibiting DHFR, Methotrexate effectively blocks DNA synthesis in rapidly dividing cancer cells. This mechanism is particularly effective in treating cancers such as leukemia, lymphoma, and certain solid tumors.

**Why Each Wrong Option is Incorrect**
* **Option A:** Cisplatin is a platinum-based chemotherapeutic agent that works by cross-linking DNA, leading to apoptosis in cancer cells. It does not target the folate pathway.
* **Option B:** 5-Fluorouracil is an antimetabolite that inhibits thymidylate synthase, but it does not directly target the folate pathway like Methotrexate.
* **Option D:** Vincristine is a vinca alkaloid that disrupts microtubule formation, leading to cell cycle arrest, but it does not affect folate metabolism.

**Clinical Pearl / High-Yield Fact**
Methotrexate is a classic example of an antifolate drug, and its mechanism of action is a crucial concept in oncology. Remember that Methotrexate can cause folate deficiency as a side effect, which can be managed with folic acid supplements.

**Correct Answer: C. Methotrexate**