Which of the following is diagnostic of acute hepatitis B infection?

Because HBcAg is intracellular and, when in the serum, sequestered within an HBsAg coat, naked core paicles do not circulate in serum and, therefore, HBcAg is not detectable routinely in the serum of patients with HBV infection. By contrast, anti-HBc is readily demonstrable in serum, beginning within the first 1-2 weeks after the appearance of HBsAg and preceding detectable levels of anti-HBs by weeks to months. During the "gap" or "window" period, anti-HBc may represent the only serologic evidence of current or recent HBV infection, and blood containing anti-HBc in the absence of HBsAg and anti-HBs has been implicated in the development of transfusion-associated hepatitis B. (Harrison's Principles of internal medicine, 20 th edition, page 2540 )