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

A. Tuberculosis
B. Acute lymphoblastic leukemia
C. Lupus
D. Psoriasis

**Core Concept:** Methotrexate is an antifolate drug with immunosuppressive, anti-proliferative, and anti-inflammatory effects. It is a competitive inhibitor of dihydrofolate reductase (DHFR), an enzyme involved in the synthesis of pyrimidines and purines, which are essential for DNA synthesis and cell division.

**Why the Correct Answer is Right:** High-dose methotrexate is used to treat Acute Lymphoblastic Leukemia (ALL) primarily due to its ability to suppress the proliferation of rapidly dividing malignant lymphocytes. The high dose ensures a rapid fall in blood counts, leading to a significant decrease in tumor burden.

**Why Each Wrong Option is Incorrect:**

A. Tuberculosis: High-dose methotrexate is not indicated for tuberculosis treatment. Instead, antibiotics like isoniazid, rifampicin, pyrazinamide, and ethambutol are used.

B. Lupus: While methotrexate is used in the treatment of Systemic Lupus Erythematosus (SLE), it is typically used at lower doses and not specifically for its immunosuppressive effect.

C. Psoriasis: Methotrexate is used for the treatment of psoriasis, but its mechanism of action is not directly related to high-dose administration. It is used for psoriasis due to its immunosuppressive and anti-inflammatory effects.

D. Psoriasis: Methotrexate is used for the treatment of psoriasis, but its mechanism of action is not directly related to high-dose administration. It is used for psoriasis due to its immunosuppressive and anti-inflammatory effects.

**Clinical Pearl:** High dose methotrexate can be used in the treatment of osteosarcoma (bone cancer), but it is not a standard treatment for this condition. Instead, high-dose chemotherapy, often including methotrexate, is used as a part of multidrug regimens for osteosarcoma treatment.

**Why Each Wrong Option is Incorrect:**

A. Osteosarcoma: While methotrexate is used in the treatment of osteosarcoma, high-dose methotrexate is not the primary treatment for this condition. Standard treatments for osteosarcoma include surgery, radiation therapy, and multidrug chemotherapy regimens, which may include methotrexate.

B. Multiple Myeloma: Although methotrexate is used in the treatment of multiple myeloma, high-dose methotrexate is not the primary treatment for this condition. Multiple myeloma is usually treated with chemotherapy regimens that include agents like bortezomib, thalidomide, and dexamethasone, rather than high-dose methotrexate.

C. Chronic Myeloid Leukemia (CML): High-dose methotrexate is not the primary treatment for Chronic Myeloid Leukemia (CML