This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Virology Pa-2 (RNA Virus Pa-1,2 & Miscellaneous Viruses), Subject name: Microbiology
[{'content': 'A patient comes to hospital with a history of sore throat, diarrhea 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: The p24 antigen can be detected during window period, during which antibodies are not demonstrated in the serum. So, used for early detection. The p24 antigen appears within 2 weeks (average 16 days) after infection. So, it is the best investigation to rule out HIV among the options given above. Note: Nucleic acid test (NAT) is the earliest test to become positive, (even before p-24 antigen detection) Test Average period after infection when test becomes positive Nucleic acid test p24 antigen test Antibodies test 12 days 16 days 22 days Western blot: It detects the HIV antibodies. A lymph node biopsy can confirm a diagnosis of AIDS-related lymphoma.', 'role': 'assistant'}]