## **Core Concept**
The infectivity of Hepatitis B Virus (HBV) is primarily indicated by the presence of certain viral markers in the blood. HBV has several antigens and genetic material that can be detected during different phases of the infection. The **HBeAg (Hepatitis B e-antigen)** is particularly significant as it is a marker of viral replication and high infectivity.

## **Why the Correct Answer is Right**
The correct answer, **HBeAg**, is a marker that signifies active viral replication. When HBV is actively replicating, it produces HBeAg, which is secreted into the bloodstream. The presence of HBeAg indicates that the virus is not only present but also actively replicating, making the individual highly infectious. This is because HBeAg positivity correlates with high levels of viral DNA in the blood, indicating active viral replication.

## **Why Each Wrong Option is Incorrect**
- **Option A: Anti-HBc (Antibody to Hepatitis B core antigen)**. While anti-HBc is a marker of current or past HBV infection, it does not specifically indicate infectivity. It can be present in both acute and chronic infections.
- **Option B: HBsAg (Hepatitis B surface antigen)**. This is a marker of HBV infection but does not directly indicate the level of infectivity. It can be present in both acute and chronic infections and does not correlate directly with viral replication.
- **Option D: Anti-HBs (Antibody to Hepatitis B surface antigen)**. This indicates immunity to HBV, either through vaccination or past infection, and does not signify infectivity.

## **Clinical Pearl / High-Yield Fact**
A crucial clinical pearl is that the presence of **HBeAg** along with **HBsAg** and high viral DNA levels indicates high infectivity and active viral replication. This information is critical for managing patients with HBV, determining the need for antiviral therapy, and assessing the risk of transmission.

## **Correct Answer: C. HBeAg**