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. P 24 antigen assay, B. ELISA, C. Western blot, D. Lymphnode biopsy', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Laboratory diagnosis of HIV infection\nLaboratory diagnosis is made by:\nA. Antibody detection\n\nIt is the most common method used for diagnosis of HIV infection.\nELISA is the most sensitive test and therefore is the screening test of choice.\nWestern blot is the most specific (Confirmatory) test and therefore is the’gold standard’ for diagnosis.\nAntibodies test usually nucleic acid or antigen.\n\nB. Detection of HIV nucleic acid or antigen\n\nP24 antigen detection by ELISA can be done 16 days after infection (earlier than antibody detection).\nNucleic acid detection tests are the earliest to become positive (12 days after infection), Most commonly used test is reverse transcriptase PCR, (RT-PCR), other methods used are DNA-PCR, RNA-PCR, RNA-bDNA (branched DNA), and RNA-NASBA (nuclei acid sequenced based assay).\n\nC. Virus isolation\n\nCo-cultivation of the patient’s lymphocyte with uninfected lymphocytes in the presence of IL-2.\nNot suitable as a routine diagnostic procedure.\n\nIn infected individual HIV can be isolated from\n\nBlood\nSemen\nSaliva\nUrine\nLymphocytes\nCervical secretions\nTears\nBreast milk', 'role': 'assistant'}]