Which serological marker is most diagnostic of ACUTE hepatitis B infection?

Anti-HBc IgM (IgM antibody to hepatitis B core antigen) is the most specific serological marker for acute HBV infection. It appears approximately 2 weeks after HBsAg during the acute phase and persists for approximately 6 months before being replaced by IgG. Its presence indicates recent (acute) infection. HBsAg indicates current infection (acute or chronic). Anti-HBc IgG indicates past exposure (resolved or chronic infection). Anti-HBs indicates immunity (from resolved infection or vaccination). In the 'window period' (when HBsAg has cleared but Anti-HBs has not yet appeared), Anti-HBc IgM may be the only marker of acute HBV infection.