**Question:** High dose methotrexate is used for the treatment of:

A. Malignant tumors, such as osteogenic sarcoma
B. Acute lymphoblastic leukemia (ALL)
C. Chronic lymphocytic leukemia (CLL)
D. Acute myeloid leukemia (AML)

**Core Concept:**
Methotrexate is a folic acid analog and antifolate medication used as an antineoplastic agent, immunosuppressant, and anti-inflammatory drug. It works by inhibiting dihydrofolate reductase (DHFR), a crucial enzyme involved in the synthesis of purines and pyrimidines, which are essential components of DNA and RNA. This results in cell cycle arrest and apoptosis in rapidly dividing cells.

**Why the Correct Answer is Right:**
High dose methotrexate is typically used in the treatment of osteogenic sarcoma, a rare malignant bone tumor. Methotrexate's ability to effectively target rapidly dividing cells makes it an effective drug for treating osteogenic sarcoma, which involves the uncontrolled proliferation of mesenchymal cells in the bone.

**Why Each Wrong Option is Incorrect:**
A. Malignant tumors, such as osteogenic sarcoma: Correct
B. Acute lymphoblastic leukemia (ALL): Methotrexate is not the first-line treatment for ALL, which is primarily treated with chemotherapy regimens like the Berlin-Frankfurt-Münster (BFM) protocol. Methotrexate plays a role in ALL treatment, but it is not the primary agent for this type of leukemia.
C. Chronic lymphocytic leukemia (CLL): Methotrexate is not the primary treatment for CLL, which is typically managed with chemotherapeutic agents like fludarabine, bendamustine, or chlorambucil. Methotrexate has some activity in CLL but is not the primary agent.
D. Acute myeloid leukemia (AML): Methotrexate is not the primary treatment for AML, which is managed with cytotoxic agents like cytarabine, daunorubicin, or idarubicin. Methotrexate plays a supportive role in AML treatment, but it is not the primary agent.

**Clinical Pearls:**
1. High-dose methotrexate is a cornerstone of osteogenic sarcoma treatment, making it a crucial component of multimodal treatment regimens including surgery, radiation, and other chemotherapy drugs.
2. Although methotrexate is effective in treating certain types of leukemia (e.g., ALL, CLL, and AML), it is not the primary agent for these conditions.
3. Methotrexate is used as a supportive treatment along with other cytotoxic agents in AML treatment, but it is not the primary drug.
4. When using methotrexate in treatment, pay attention to its potential side effects, including acute renal failure, mucositis, and neurotoxicity. These side effects can be minimized by administering leucovorin (folinic acid) as a rescue agent to counteract methotrexate toxicity.