This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Infection, Subject name: Medicine
A patient develops hepatosplenomegaly, lymphadenopathy following sexual contact, 3 weeks back. The best test to rule out HIV infection is : 
Select and explain. 
A. ELISA, B. Western blot, C. P24Ag, D. Lymph node biopsy
C: P24Ag
Answer: C: P24Ag 
Explanation: Ans. is 'c' P24 Ag. "Antibodies to HIV usually appear within 6 weeks and almost invariably within 12 weeks of primary infection." - Harrison,15/eDetection of these Antibodies forms the basis of most diagnostic screening tests for HIV infection. Such as ELISA & Western blot test.In the question the patient is giving H/O sexual contact just 3 weeks back. So there is strong probability of that Antibodies might not have developed against HIV. Now the diagnosis of HIV in this patient depends upon the direct detection of HIV or one of its components.The simplest of the direct detection test is the p24 Antigen Capture Assay.