## **Core Concept**
Methotrexate is an antifolate drug used in the treatment of various cancers and autoimmune diseases. It works by inhibiting dihydrofolate reductase (DHFR), an enzyme crucial for DNA synthesis and cell replication. Resistance to methotrexate can arise through several mechanisms.

## **Why the Correct Answer is Right**
The correct answer, **C. Increased DHFR enzyme**, is right because one of the primary mechanisms of methotrexate resistance is the overexpression of the target enzyme itself, dihydrofolate reductase (DHFR). When DHFR is overexpressed, more methotrexate is required to inhibit the enzyme and achieve the same level of cytotoxicity. This is a well-documented mechanism of resistance in cancer cells.

## **Why Each Wrong Option is Incorrect**
- **Option A: Decreased DHFR enzyme** - This would actually increase the sensitivity of cells to methotrexate, not lead to resistance, as there would be less DHFR enzyme for methotrexate to inhibit.
- **Option B: Increased folate transport** - While alterations in folate transport can affect methotrexate activity (methotrexate enters cells through the reduced folate carrier), increased transport would not typically lead to resistance; decreased transport is a known resistance mechanism.
- **Option D: Decreased polyglutamation** - Polyglutamation is a process where methotrexate is converted into its active, polyglutamated form inside cells, which enhances its retention and efficacy. Decreased polyglutamation would indeed lead to resistance, but the correct answer focuses on the direct interaction with the drug's target.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that methotrexate resistance can be overcome or managed by using higher doses of the drug, by administering folinic acid (leucovorin) to rescue normal cells from methotrexate toxicity (though not in the context of resistance), or by combining methotrexate with other drugs that can overcome resistance mechanisms.

## **Correct Answer: C. Increased DHFR enzyme**