**Core Concept**
Dihydrofolate reductase (DHFR) is an enzyme crucial for the synthesis of tetrahydrofolate, a necessary cofactor in one-carbon transfer reactions. Inhibiting DHFR can disrupt DNA synthesis and repair, making it a target for cancer chemotherapy and certain infections.

**Why the Correct Answer is Right**
Methotrexate is a well-known DHFR inhibitor that competes with dihydrofolate for the active site of the enzyme, thereby preventing the reduction of dihydrofolate to tetrahydrofolate. This action inhibits the synthesis of DNA, RNA, and proteins, ultimately leading to cell death. Methotrexate is commonly used in the treatment of certain cancers, autoimmune diseases, and severe psoriasis.

**Why Each Wrong Option is Incorrect**
**Option A:** Aminopterin is another DHFR inhibitor, but it is not as commonly used as methotrexate. While it is also effective, methotrexate is more widely available and has a more favorable side-effect profile.
**Option B:** Folic acid is actually a precursor to tetrahydrofolate, and supplementing with it can counteract the effects of DHFR inhibitors like methotrexate.
**Option C:** Cyclophosphamide is an alkylating agent used in chemotherapy, but it does not inhibit DHFR.

**Clinical Pearl / High-Yield Fact**
Methotrexate can cause megaloblastic anemia, a condition characterized by large, immature red blood cells, due to its inhibition of DHFR and subsequent disruption of DNA synthesis.

**Correct Answer:** C. Methotrexate.