This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
[{'content': 'Leflunomide acts on: \nSelect and explain. \nA. Purine synthesis, B. Pyrimidine synthesis, C. Cell membrane synthesis, D. None of the above', 'role': 'user'}, {'content': "Answer: B \nExplanation: Leflunomide is metabolized to an active metabolite that acts to inhibit dihydroorotate dehydrogenase, an essential enzyme in the pyrimidine biosynthetic pathway. Its predominant action is to inhibit the proliferation of T lymphocytes. Leflunomide has been shown to control the signs and symptoms of RA and to slow the progression of joint damage as effectively as methotrexate. Leflunomide can be given alone or with methotrexate and is the most frequently employed immunosuppressive agent used to treat patients with Rheumatoid ahritis. Ref: Harrison's principle of internal medicine 17th edition, chapter 302.", 'role': 'assistant'}]