**Core Concept:** Hepatitis B virus (HBV) infection is a common viral disease affecting the liver. It can lead to acute or chronic infection, with potential complications like cirrhosis and hepatocellular carcinoma. Hepatitis B surface antigen (HBsAg) is an early marker of HBV infection and is usually detected in the acute phase of infection. Anti-HBs, also known as hepatitis B surface antibody, is produced in response to vaccination or previous infection and indicates immunity against HBV. Anti-HBc, hepatitis B core antibody, is produced during acute infection and persists after clearance of the virus. Chronic hepatitis B carriers usually have both HBsAg and anti-HBc.

**Why the Correct Answer is Right:** The correct answer, anti-HBc, is always present in chronic hepatitis B carriers. In the acute phase of HBV infection, HBsAg is detected, followed by the production of anti-HBs and anti-HBc. Chronic carriers have both HBsAg and anti-HBc, indicating the persistence of the virus and active infection. Chronic carriers may lose HBsAg but will always have anti-HBc.

**Why Each Wrong Option is Incorrect:**

A. HBsAg: Present in acute infection but not in chronic carriers, as it indicates clearance of the virus.
B. Anti-HBs: Present in immune individuals but not in chronic carriers, as it indicates immunity achieved via vaccination or previous infection.
C. Both HBsAg and anti-HBs: These markers are typically found in acute hepatitis B infection, not in chronic carriers.
D. Both HBsAg and anti-HBs: Similar to option C, this combination is found in acute infection, not chronic carriers.

**Clinical Pearl:**

A hepatitis B surface antigen (HBsAg) test is crucial for diagnosing acute HBV infection, while anti-HBc is essential for identifying chronic carriers. Combining both tests allows for accurate differentiation between acute and chronic infection, ensuring appropriate management and surveillance strategies.