## Core Concept
High-dose methotrexate is a chemotherapy regimen used for treating certain types of cancer. Methotrexate is an antifolate drug that inhibits dihydrofolate reductase (DHFR), an enzyme important for DNA synthesis and cell replication. This mechanism is particularly effective against rapidly dividing cells, such as cancer cells.

## Why the Correct Answer is Right
The correct answer, , indicates that high-dose methotrexate is used for the treatment of certain malignancies. High-dose methotrexate is specifically utilized in the treatment of **osteosarcoma**, **Ewing's sarcoma**, and **rhabdomyosarcoma**, among other cancers. It is also used in the treatment of **leukemias**, especially acute lymphoblastic leukemia (ALL). The high dose is required to achieve cytotoxic concentrations of methotrexate in the tumor cells.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because, although methotrexate is used in autoimmune diseases like rheumatoid arthritis and psoriasis, the question specifically asks about high-dose methotrexate, which is not typically used for these conditions.
- **Option B:** This option is incorrect because, while methotrexate can be used in some autoimmune diseases, high-dose methotrexate is not standard treatment for conditions like Crohn's disease or ulcerative colitis.
- **Option D:** This option is incorrect because high-dose methotrexate is not primarily used for treating infections.

## Clinical Pearl / High-Yield Fact
A key point to remember is that when administering high-dose methotrexate, **leucovorin rescue** is often used to mitigate toxicity, particularly to the bone marrow and gastrointestinal tract. Leucovorin provides a form of folate that can bypass the inhibition caused by methotrexate, allowing normal cells to recover.

## Correct Answer: C.