**Core Concept**
Methotrexate is a chemotherapeutic agent and immune system suppressant that can cause toxicity through folate antagonism, leading to an imbalance in the body's use of folate and, consequently, DNA synthesis. This toxicity can be prevented or mitigated by administering compounds that counteract its effects.

**Why the Correct Answer is Right**
To prevent methotrexate toxicity, folic acid is administered concurrently. This is because folic acid can bypass the enzyme dihydrofolate reductase (DHFR), which is inhibited by methotrexate. By providing an alternative route for folate to enter the cell, folic acid supplementation helps to mitigate the toxic effects of methotrexate on rapidly dividing cells, such as bone marrow and gastrointestinal mucosa.

**Why Each Wrong Option is Incorrect**
**Option A:** Leucovorin is actually a rescue medication used to treat methotrexate toxicity, not prevent it.
**Option B:** Folinic acid is another name for leucovorin, and it serves the same purpose as leucovorin in treating methotrexate toxicity.
**Option C:** This option is blank, so we'll skip it.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that folic acid supplementation is initiated 24 hours before starting methotrexate and continued for 1-2 weeks after the completion of the chemotherapeutic regimen to prevent toxicity.

**Correct Answer:** D. Folic acid