This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Human Immunodeficiency Virus, Subject name: Microbiology
A patient comes to hospital with a history of sore throat, diarrhea and sexual contact 2 weeks before. The best investigation to rule out HIV is: 
Select and explain. 
A. p24 antigen assay, B. ELISA, C. Western blot, D. Lymph node biopsy
A: p24 antigen assay
Answer: A: p24 antigen assay 
Explanation: Ans. is 'a' i.e., p. 24 antigen assay (Ref: Ananthanarayan, 9th/e, p. 577 - 578 and 8th/e, p. 576)* p24 antigen is the earliest virus marker to appear in blood and is the one tested for.* It may be detectable in blood after about 2 weeks.Remember: Nucleic acid tests (RT - PCR, BONA, PCR) are the earliest test to become positive (even before p24 antigen detection).TestPositive after infectionNucleic acid test12 daysp24 antigen test16 daysAntibodies test22 days