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': 'A patient comes to hospital with a history of sore throat, diarrhoea and sexual contact 2 weeks before. The best investigation to rule out HIV is \nSelect and explain. \nA. P24 antigen assay, B. ELISA, C. Western blot, D. Lymph node biopsy', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]