**Question:** Marker for infectivity of serum in Hepatitis B is

A. Hepatitis B surface antigen (HBsAg)
B. Hepatitis B e antigen (HBeAg)
C. Hepatitis B core antibody (anti-HBc)
D. Hepatitis B surface antibody (anti-HBs)

**Correct Answer:**

A. Hepatitis B surface antigen (HBsAg)

**Core Concept:**

Hepatitis B is a viral infection caused by the Hepatitis B virus (HBV), which affects the liver and can lead to chronic liver disease, cirrhosis, and hepatocellular carcinoma. The infectivity of the serum in Hepatitis B is determined by the presence of specific markers.

In the context of HBV infection, the infectivity marker refers to the presence of Hepatitis B surface antigen (HBsAg). HBsAg is produced by the virus and indicates the active replication of the virus within the infected individual.

**Why the Correct Answer is Right:**

HBsAg is the correct answer because it is produced during the active replication of HBV in the infected individual. This marker indicates the presence of infectious virus particles in the blood, making the serum highly contagious.

**Why Each Wrong Option is Incorrect:**

A. Hepatitis B e antigen (HBeAg): HBeAg is produced during the early phase of infection, suggesting high viral replication but does not indicate infectivity directly.

B. Hepatitis B core antibody (anti-HBc): Anti-HBc is produced during the recovery phase of an infection, indicating past infection but not current infectivity.

C. Hepatitis B surface antibody (anti-HBs): Anti-HBs is produced during the recovery phase of infection, indicating past infection but does not indicate current infectivity.

**Clinical Pearl:**

The presence of HBsAg is crucial for diagnosing acute HBV infection and assessing the infectivity of the patient's blood. Identifying HBsAg-positive individuals can help implement preventive measures, such as vaccinating contacts or administering post-exposure prophylaxis in high-risk individuals. In contrast, detecting HBsAg-negative individuals can indicate recovery from the infection, suggesting they are not contagious. Understanding these markers is essential for managing HBV infection and preventing its transmission.