Which serologic marker is the FIRST to appear in acute hepatitis B infection?

Correct answer: C. HBsAg (hepatitis B surface antigen)

• A. Anti-HBe appears as the infection resolves and correlates with declining viral replication and reduced infectivity.
• B. Anti-HBs appears LATE, after HBsAg clears, and signals recovery and immunity. It is also the only marker present after vaccination.
• C. Correct. HBsAg is the EARLIEST detectable marker, appearing 1–10 weeks after exposure and before symptoms or transaminase elevation. Its presence indicates active infection (acute or chronic) and infectivity. Persistence beyond 6 months defines chronic hepatitis B.
• D. IgG anti-HBc appears later and persists for life, indicating past or chronic infection. It is IgM anti-HBc that marks recent/acute infection.

Note: Distinguishing acute from chronic: BOTH are HBsAg-positive. It is IgM ANTI-HBc that identifies ACUTE infection, and it is the only marker present during the 'window period' when HBsAg has cleared but anti-HBs has not yet appeared. Vaccination gives anti-HBs ONLY — no anti-HBc.