**Core Concept**
Methotrexate toxicity occurs due to the accumulation of the drug or its metabolites, leading to folate deficiency and interference with DNA synthesis. The treatment aims to replenish folate stores and counteract the toxic effects of methotrexate.

**Why the Correct Answer is Right**
Leucovorin (also known as folinic acid) is used to treat methotrexate toxicity. It is a reduced form of folate that can bypass the enzyme dihydrofolate reductase, which is inhibited by methotrexate. Leucovorin can rescue normal cells from the toxic effects of methotrexate by providing a source of one-carbon units for DNA synthesis. This is particularly important in situations where methotrexate is used at high doses or in patients with impaired renal function.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not address the specific mechanism of methotrexate toxicity.
**Option B:** This option is incorrect because it is not a standard treatment for methotrexate toxicity.
**Option C:** This option is incorrect because it does not specifically counteract the toxic effects of methotrexate.

**Clinical Pearl / High-Yield Fact**
Leucovorin should be administered within 24 hours of methotrexate exposure to be effective in treating toxicity. This is a critical time frame for intervention, and prompt administration of leucovorin can significantly reduce the risk of severe toxicity.

**Correct Answer: C. Leucovorin**