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
A patient presented with fever, cervical lymphadenopathy and night sweats four weeks after unprotected sexual contact. Next investigation 
Select and explain. 
A. p24, B. ELISA, C. CD4, D. HIV PCR
A: p24
Answer: A: p24 
Explanation: 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.