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
Early diagnosis of acute hepatitis-B infection is made by- 
Select and explain. 
A. Presence of HbeAg in serum, B. Presence of IgM anti-Hbc in serum, C. Presence HbsAg in serum, D. Presence of IgG anti-HBc in serum
C: Presence HbsAg in serum
Answer: C: Presence HbsAg in serum 
Explanation: After a person is infected with HBV, the first virologic marker detectable in serum within 1-12 weeks, usually between 8-12 weeks, is HBsAg. Circulating HBsAg precedes elevations of serum aminotransferase activity and clinical symptoms by 2-6 weeks and remains detectable during the entire icteric or symptomatic phase of acute hepatitis B and beyond. In typical cases, HBsAg becomes undetectable 1-2 months after the onset of jaundice and rarely persists beyond 6 months. After HBsAg disappears, antibody to HBsAg (anti-HBs) becomes detectable in serum and remains detectable indefinitely thereafter. (Harrison's Principles of internal medicine, 20 th edition, page 2540)