## Core Concept
The question tests the understanding of managing methotrexate toxicity, which is a chemotherapeutic agent used in the treatment of various cancers and autoimmune diseases. Methotrexate works by inhibiting dihydrofolate reductase (DHFR), an enzyme crucial for DNA synthesis and cell replication. Toxicity from methotrexate can occur due to its mechanism of action or due to impaired renal function, leading to elevated levels of the drug.

## Why the Correct Answer is Right
The correct answer, **C. Leucovorin**, is used to treat methotrexate toxicity. Leucovorin, also known as folinic acid, is a form of folate that can bypass the inhibition of DHFR caused by methotrexate. By administering leucovorin, it provides a source of reduced folate for DNA synthesis, thereby rescuing normal cells from the toxic effects of methotrexate without interfering with the therapeutic effects of methotrexate on cancer cells, which are often more sensitive to methotrexate's effects.

## Why Each Wrong Option is Incorrect
- **Option A: Folic acid** is incorrect because, although it is a precursor to folinic acid, it needs to be converted to folinic acid by dihydrofolate reductase, an enzyme inhibited by methotrexate. Therefore, administering folic acid would not effectively counteract methotrexate toxicity immediately.
- **Option B: Vitamin B12** is incorrect because it is not directly involved in the pathway affected by methotrexate. Vitamin B12 is crucial for the synthesis of methionine and S-adenosylmethionine but does not directly counteract methotrexate's mechanism of action.
- **Option D: Fosphenytoin** is incorrect because it is an antiepileptic medication and has no role in the management of methotrexate toxicity.

## Clinical Pearl / High-Yield Fact
A key point to remember is that leucovorin "rescue" is a critical strategy in managing methotrexate toxicity, especially in patients with elevated methotrexate levels or those showing signs of toxicity. The timing and dose of leucovorin are crucial and depend on the severity of the toxicity and the methotrexate levels.

## Correct Answer: C. Leucovorin