Which hepatitis marker would indicate a new infection if found in a patient?

The presence of HBsAg (Hepatitis B surface Antigen) indicates an active infection with the Hepatitis B virus. When HBsAg is detected in a patient, it signifies that the person is currently infected, which can be either an acute or chronic stage of the infection. In the context of a new infection, HBsAg would typically be one of the first markers to appear, often before the onset of symptoms and before the appearance of antibodies.

This biomarker is critical in identifying individuals who are infectious and enables healthcare providers to assess the risk of transmission and the need for immediate medical intervention. In the acute phase of Hepatitis B infection, HBsAg will be present and can then be followed over time to monitor the infection's progression or resolution, with the eventual development of antibodies such as Anti-HBs if the infection resolves favorably.

Although other markers like Anti-HBc can also indicate past or current infections, they are less specific for identifying a new infection than HBsAg. Therefore, the correct choice indicating a new Hepatitis B infection is, without doubt, HBsAg.