**Core Concept**
5-Fluorouracil (5-FU) is a chemotherapeutic agent used to treat various cancers, including colorectal, breast, and stomach cancers. It belongs to the class of pyrimidine analogs, which interfere with DNA synthesis.

**Why the Correct Answer is Right**
5-FU exerts its antineoplastic effect by inhibiting thymidylate synthase (TS), an enzyme crucial for DNA synthesis and repair. Once inside the cancer cell, 5-FU is converted into its active metabolite, 5-fluoro-2'-deoxyuridine-5'-monophosphate (FdUMP). FdUMP binds to TS, forming a stable complex that inactivates the enzyme. This leads to a reduction in thymidine synthesis, causing DNA strand breaks and ultimately cell death. The inhibition of TS also results in the accumulation of 2'-deoxyuridine-5'-monophosphate (dUMP), which is converted into thymidine by the enzyme thymidine kinase. However, the presence of FdUMP- TS complex prevents this conversion, further contributing to the depletion of thymidine and DNA synthesis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the mechanism of action of 5-FU.
**Option B:** Although 5-FU is a pyrimidine analog, its primary mechanism of action is not through the inhibition of dihydrofolate reductase (DHFR), which is a target for other chemotherapeutic agents like methotrexate.

**Clinical Pearl / High-Yield Fact**
A common side effect of 5-FU is hand-foot syndrome (palmar-plantar erythrodysesthesia), which can be caused by the accumulation of 5-FU and its metabolites in the skin. This condition is often managed with dose adjustments or the use of supportive measures.

**Correct Answer: C. Inhibition of thymidylate synthase.**