**Core Concept**
Leucovorin, also known as folinic acid, is a form of folic acid used in medicine to counteract the toxic effects of certain chemotherapy medications, particularly those that inhibit dihydrofolate reductase (DHFR). This is crucial in the treatment of various cancers, such as colorectal cancer.

**Why the Correct Answer is Right**
Leucovorin works by bypassing the inhibition of DHFR by drugs like methotrexate, which are used to treat cancer. By providing a source of reduced folate, leucovorin enables the synthesis of purines and thymidylate, necessary for DNA replication. This helps to counteract the toxic effects of methotrexate on rapidly dividing cells, such as cancer cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the use of leucovorin in reducing toxicity.
* **Option B:** While leucovorin is used in combination with fluorouracil (5-FU), it is not used to decrease the toxicity of 5-FU itself, but rather to enhance its effectiveness.
* **Option C:** This option is incorrect as leucovorin is not used to decrease the toxicity of this medication.

**Clinical Pearl / High-Yield Fact**
Leucovorin is often used in high doses to rescue patients from the toxic effects of methotrexate, a chemotherapy medication used to treat various types of cancer. This is a critical aspect of cancer treatment, as it enables the continued administration of methotrexate while minimizing its toxic effects.

**Correct Answer: D. Methotrexate**