**Core Concept**
Leucovorin, also known as **folinic acid**, is used to decrease the toxicity of certain chemotherapeutic agents by counteracting their adverse effects on the body. This is particularly relevant in the context of **antineoplastic therapy**, where the goal is to minimize harm to healthy cells while maximizing the therapeutic effect on cancer cells. Leucovorin acts as a **rescue agent**.

**Why the Correct Answer is Right**
The correct answer is related to **methotrexate**, a chemotherapeutic agent that inhibits **dihydrofolate reductase**, an enzyme necessary for the synthesis of **tetrahydrofolate**. Leucovorin provides an external source of folinic acid, which can bypass this inhibition, thus reducing methotrexate's toxicity on healthy cells, especially the **gastrointestinal mucosa** and **bone marrow**.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because, although leucovorin is involved in folate metabolism, its primary use in decreasing toxicity is specific to certain drugs, not broadly to all **antifolates**.
**Option B:** Incorrect as it is not directly related to the primary use of leucovorin in decreasing drug toxicity.
**Option C:** Incorrect because, while leucovorin is crucial in supporting **thymidylate synthesis**, this option does not specify the context of reducing drug toxicity.
**Option D:** Incorrect as leucovorin's role is more specific than just supporting general **cellular metabolism**.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that leucovorin is used to "rescue" patients from the toxic effects of high-dose **methotrexate** therapy, highlighting its critical role in **oncology**.

**Correct Answer:** D. Methotrexate.