**Core Concept**
Methotrexate is a chemotherapeutic agent used in the treatment of various types of leukemia, including its administration for central nervous system (CNS) prophylaxis. CNS prophylaxis involves the use of chemotherapy to prevent the development of leukemia cells in the CNS. Methotrexate acts as an antimetabolite, inhibiting dihydrofolate reductase (DHFR) enzyme, which is essential for DNA synthesis.

**Why the Correct Answer is Right**
In acute lymphoblastic leukemia (ALL), the risk of CNS involvement is significantly higher compared to other types of leukemia. As a result, methotrexate is administered intrathecally as part of CNS prophylaxis to prevent the development of leukemic cells in the CNS. The mechanism of action of methotrexate involves the inhibition of DHFR, which leads to the depletion of tetrahydrofolate, a precursor for DNA synthesis. This results in the inhibition of DNA synthesis and cell division, ultimately leading to cell death.

**Why Each Wrong Option is Incorrect**
**Option A:** Although methotrexate is used in the treatment of various other types of leukemia, its administration for CNS prophylaxis is not a standard practice in chronic lymphocytic leukemia (CLL).
**Option B:** Methotrexate is not typically used for CNS prophylaxis in acute myeloid leukemia (AML).
**Option C:** Methotrexate is not administered for CNS prophylaxis in chronic myeloid leukemia (CML).

**Clinical Pearl / High-Yield Fact**
Methotrexate is also used in the treatment of certain autoimmune diseases, such as rheumatoid arthritis, due to its immunosuppressive properties.

**Correct Answer:** C.