## **Core Concept**
The question tests the understanding of serological markers of Hepatitis B virus (HBV) infection and their clinical implications. Hepatitis B is a viral infection that affects the liver, and its infectivity is determined by the presence of specific viral antigens and antibodies in the blood.

## **Why the Correct Answer is Right**
The correct answer, **HBeAg (Hepatitis B e-antigen)**, is a marker of HBV replication and infectivity. It is an envelope protein that is secreted from infected hepatocytes and appears in the blood when the virus is actively replicating. The presence of HBeAg indicates that the virus is actively replicating and the patient is highly infectious.

## **Why Each Wrong Option is Incorrect**
* **Option A: HBsAg (Hepatitis B surface antigen)** - While HBsAg is a marker of current HBV infection, its presence does not necessarily indicate active viral replication or high infectivity. It can be present in both acute and chronic infections.
* **Option B: Anti-HBc (Antibody to Hepatitis B core antigen)** - This is a marker of current or past HBV infection but does not indicate infectivity. It appears during the window period between the disappearance of HBsAg and the appearance of anti-HBs.
* **Option D: Anti-HBs (Antibody to Hepatitis B surface antigen)** - This is a marker of immunity to HBV, either due to vaccination or past infection. Its presence indicates a reduced risk of infection and is not a marker of infectivity.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the presence of **HBeAg** correlates with high viral replication and infectivity, whereas the presence of **anti-HBe** (antibody to HBeAg) often indicates a lower viral replication phase but can still be infectious due to the presence of HBV DNA.

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