Which positive test result for hepatitis B would indicate that an exposure to HBV has occurred in the source patient?

• A. HBV surface antigen (HBsAg)
• B. Anti-HBs
• C. Anti-HBc
• D. HBc IgM

Answer: (A)

Understanding hepatitis B serology requires linking what each marker means to infection status. The surface antigen (HBsAg) appears in the blood when the virus is actively replicating. Its presence signals an active HBV infection and that the person is contagious, which is the clearest indication that exposure to HBV has occurred and led to infection in the source patient.

In contrast, antibodies like anti-HBs indicate immunity (either from vaccination or recovery), not current infection. Anti-HBc shows that the person has been exposed to HBV at some point, but by itself it doesn’t confirm an active infection. HBc IgM points to a recent acute infection, but the strongest marker for ongoing exposure with infection is the surface antigen.