This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Oncology, Subject name: Medicine
[{'content': 'A young man with leukemia is treated with methotrexate. Which of the following is the mechanism of action of this drug? \nSelect and explain. \nA. preventing absorption of folic acid, B. inhibiting dihydrofolate reductase, C. preventing formation of messenger ribonucleic acid (mRNA), D. forming a cytotoxic metabolite', 'role': 'user'}, {'content': 'Answer: B \nExplanation: The most likely mode of action of methotrexate is by tightly binding dihydrofolate reductase (DHFR), which maintains the intracellular folate pool in its fully reduced form as tetrahydrofolates. These compounds are required in the de novo synthesis of pyrimidines and purines.', 'role': 'assistant'}]