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
Leucovorin is used to decrease the toxicity of 
Select and explain. 
A. Methotrexate, B. 6-Mercaptopurine, C. Thio-TEPA, D. Cytosine arabinoside
A: Methotrexate
Answer: A: Methotrexate 
Explanation: Ref: Katzung 10th/887; KDT 6/e 823 Toxicity of methotrexate can be reversed by the administration of 5-formyltetrahydrofolate (leucovorin or citrovorum factor). Folic acid in the diet is in dihydrofolate form. It is ineffective in reversing methotrexate toxicity.