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: Microbiology
[{'content': 'A person has unprotected sex 3 weeks back. To rule out HIV infection the best test is: \nSelect and explain. \nA. P24 antigen assay, B. ELISA, C. Western blot, D. Lymphnode biopsy', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. a. P24 antigen assay', 'role': 'assistant'}]