## **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 dihydrofolate reductase (DHFR) enzyme**, is a well-documented mechanism of methotrexate resistance. Overexpression of the DHFR gene leads to an increased amount of DHFR enzyme, which can bind and neutralize methotrexate, thereby reducing its effectiveness. This is a common mechanism by which cancer cells develop resistance to methotrexate.

## **Why Each Wrong Option is Incorrect**
- **Option A: Decreased dihydrofolate reductase (DHFR) enzyme** - This would actually increase the sensitivity of cells to methotrexate, not lead to resistance.
- **Option B: Decreased folate transport** - While alterations in folate transport can affect methotrexate uptake and contribute to resistance, the most direct and commonly cited mechanism related to the options provided is the overexpression of DHFR.
- **Option D: Increased glutathione S-transferase** - This enzyme is involved in detoxifying various chemotherapeutic agents but is not directly related to the primary mechanism of action of methotrexate.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **methotrexate resistance can be overcome by using higher doses or by combining it with other drugs that can modulate its effects**. For example, leucovorin (folinic acid) can be used to rescue normal cells from methotrexate toxicity but is less effective in cells with DHFR overexpression.

## **Correct Answer: C. Increased dihydrofolate reductase (DHFR) enzyme**