This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Chemotherapy, Subject name: Pharmacology
High dose methotrexate is used for the treatment of: 
Select and explain. 
A. Osteosarcoma, B. Rhabdomyosarcoma, C. Retinoblastoma, D. Ewing's sarcoma
A: Osteosarcoma
Answer: A: Osteosarcoma 
Explanation: Methotrexate (Mtx) It is one of the oldest and highly efficacious antineoplastic drugs; inhibits dihydrofolate reductase (DHFRase)-blocking the conversion of dihydrofolic acid (DHFA) to tetrahydrofolic acid (THFA) which is an essential coenzyme required for one carbon transfer reactions in de novo purine synthesis and amino acid interconversions. The inhibition is pseudoirreversible because Mtx has 50,000 times higher affinity for the enzyme than the normal substrate. Methotrexate is apparently curative in choriocarcinoma.Highly effective in maintaining remission in children with acute leukaemias, but not good for inducing remission: 2.5-15 mg/day. It is also useful in other malignancies, rheumatoid ahritis, psoriasis and as immunosuppressant The use of folinic acid rescue has permitted much higher doses of Mtx and has enlarged its scope to many difficult-to-treat neoplasms ESSENTIALS OF MEDICAL PHARMACOLOGY K.D.TRIPATHI SIXTH EDITION PAGE NO:823