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 the hospital with a history of a 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: The patient is giving symptoms of the acute viral syndrome.\nThe clinically acute viral syndrome is associated with nonspecific symptoms including a sore throat, myalgia, fever, rash, weight loss and fatigue\nPatient with primary HIV infection develops the acute viral syndrome 3-6 weeks after infection, which resolves spontaneously in 2-4 weeks.\nDuring this phase, the immune response to HIV infection has not yet developed. There are no HIV specific antibody ie., no seroconversion.\nSince these phases are characterized by high level of viremia, the infection in these stages can be diagnosed by direct detection of virus or virus particles.\nA variety of laboratory tests are available for the direct detection of HIV or its components.\n\np24 Ag Assay\n\nThe simplest of the direct detection test is p24 antigen capture assay.\nDuring the first few weeks of infection before an immune response develops, there is a brisk rise in p 24 antigen. After the development of the p24 antibody, these levels decline. Thus p24 Ag assay has its greatest use as a screening test for HIV infection in patients suspected of having the acute HIV syndrome as high levels of p24 Ag are present prior to the development of antibody.\n\nDetection of HIV RNA\n\nMeasurement of HIV RNA in the plasma of patients with HIV infection is also useful in conditions where measurement of anti-HIV antibodies is not possible e.g. in acute infections and in neonatal infections.\n\nCurrently, three assays are available for the diagnosis of HIV RNA.\n\nHIV RNA by PCR\nHIV RNA by bDNA\nHIV RNA by NASBA\n\nNote: In neonates, there is the passive transmission of antibodies from the mother. Thus the antibodies cannot be used for the diagnosis in this condition as it would be impossible to tell whether the antibodies detected are acquired from the mother or they have developed as a result of infection.\nWith the use of direct detection techniques, there is a decrease in the interval between infection and detection of (window period).\nWindow period for\n\nAntibody testing → 22 days approximately\np24 Ag Assay → 16 days\nHIV RNA → 12 days.', 'role': 'assistant'}]