**Core Concept**
Methotrexate is a folate antagonist that inhibits dihydrofolate reductase (DHFR), leading to a decrease in the synthesis of tetrahydrofolate (THF), an essential cofactor for DNA synthesis and repair. This mechanism underlies its use as an antineoplastic and immunosuppressive agent.

**Why the Correct Answer is Right**
Methotrexate's mechanism of action involves the inhibition of DHFR, which is crucial for the conversion of dihydrofolate (DHF) to THF. THF is necessary for the synthesis of purines and pyrimidines, which are essential for DNA replication and repair. By blocking this pathway, methotrexate effectively prevents cancer cells from proliferating and also suppresses the immune system. This is why methotrexate is used in the treatment of various cancers, autoimmune diseases, and severe rheumatoid arthritis.

**Why Each Wrong Option is Incorrect**
**Option A:** This statement is incorrect because methotrexate is actually a folate antagonist, not a folate receptor antagonist. Folate receptor antagonists, such as pemetrexed, work by binding to folate receptors, whereas methotrexate works by inhibiting DHFR.
**Option B:** This statement is incorrect because methotrexate is actually a weak inhibitor of dihydrofolate reductase (DHFR), not a potent inhibitor. While it does inhibit DHFR, its affinity for the enzyme is relatively low compared to other DHFR inhibitors.
**Option C:** This statement is incorrect because methotrexate is actually metabolized by the enzyme cytochrome P450 3A4 (CYP3A4), not cytochrome P450 2C9 (CYP2C9). CYP3A4 is responsible for the hydroxylation of methotrexate, which leads to its inactivation.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that methotrexate can cause hepatotoxicity, particularly with high doses or prolonged use. Monitoring liver function tests and adjusting the dose accordingly is crucial to prevent liver damage. Additionally, methotrexate can interact with other medications, such as NSAIDs, which can increase the risk of hepatotoxicity.

**Correct Answer:** B.