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 presented with fever, cervical lymphadenopathy and night sweats four weeks after unprotected sexual contact. Next investigation \nSelect and explain. \nA. p24, B. ELISA, C. CD4, D. HIV PCR', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ref: PubMed* This is a possible suspicion of HIV infection.* p24 antigen is a viral protein that makes up most of the viral core.* Serum concentrations of p24 antigen are high in the first few weeks after infection; tests sensitive to p24 antigen are therefore useful for diagnosing very early infection when antibody levels are still low.', 'role': 'assistant'}]