**Core Concept**
Anti-metabolites are a class of drugs that inhibit the synthesis of nucleic acids or interfere with their function. They act by mimicking the structure of purine or pyrimidine bases, thereby competing with the natural bases for incorporation into DNA or RNA.

**Why the Correct Answer is Right**
The correct answer is an anti-metabolite because it inhibits the enzyme dihydrofolate reductase (DHFR), which is essential for the synthesis of tetrahydrofolate. Tetrahydrofolate is required for the conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP), which is necessary for DNA synthesis. By inhibiting DHFR, the correct answer prevents the production of dTMP, thereby inhibiting DNA synthesis and cell growth.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not directly inhibit DNA synthesis. Instead, it targets a different metabolic pathway.
**Option B:** This option is incorrect because it is a type of antibiotic that inhibits protein synthesis, rather than DNA synthesis.
**Option C:** This option is incorrect because it is a type of chemotherapeutic agent that induces DNA damage and apoptosis, rather than inhibiting DNA synthesis.

**Clinical Pearl / High-Yield Fact**
Anti-metabolites are a common class of chemotherapeutic agents used in the treatment of various cancers. They are often used in combination with other agents to enhance their effectiveness and reduce the risk of resistance.

**Correct Answer: C. Methotrexate.**