This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Virology, Subject name: Microbiology
[{'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. Lymphnode biopsy', 'role': 'user'}, {'content': 'Answer: A \nExplanation: following a single massive imfection the p24Ag is the earliest marker appear in blood REF:MICROBIOLOGY ANANTHA NARAYANAN NINTH EDITION PAGE.577', 'role': 'assistant'}]