**Core Concept:** Hepatitis-B infection is a viral disease caused by Hepatitis B virus (HBV). The replication of HBV in the liver cells leads to inflammation and damage, potentially resulting in chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma. Viral markers are proteins produced by the virus or host cells in response to viral infection.

**Why the Correct Answer is Right:** Hepatitis-B surface antigen (HBsAg) is a viral marker produced by the hepatitis B virus during its replication cycle. It is found in the serum of infected individuals and indicates active viral replication. HBsAg is produced at the early stage of infection and is detectable before the appearance of hepatitis B e antigen (HBeAg) and hepatitis B e antibody (HBeAb).

**Why Each Wrong Option is Incorrect:**

A. Hepatitis B core antigen (HBcAg) is a viral marker produced by the host cells in response to HBV infection. It is not specific for ongoing viral replication and is present throughout the course of infection.

B. Hepatitis B e antigen (HBeAg) is produced by the virus during the active phase of infection and indicates ongoing viral replication. However, it is not specific for active replication and can persist in the serum even when viral replication is controlled or ceased.

C. Hepatitis B e antibody (HBeAb) is produced by the host immune response after HBeAg clearance. It indicates the control or clearance of viral replication but is not specific for ongoing viral replication.

D. Hepatitis B antibody (anti-HBs) is produced by the host immune response after HBsAg clearance. It indicates past infection but does not indicate the current status of viral replication.

**Clinical Pearl:** In the clinical setting, detecting HBsAg in a patient's serum signifies an acute infection, while measuring HBeAg indicates active viral replication. Combining the results of HBsAg, HBeAg, and anti-HBe can provide a comprehensive understanding of the infection status and the likelihood of viral replication.