**Core Concept**
HBV (Hepatitis B virus) infectivity is a critical aspect of its transmission and management. The virus has a complex life cycle involving various stages of replication, transcription, and translation. Understanding HBV infectivity is essential for developing effective prevention and treatment strategies.

**Why the Correct Answer is Right**
HBV infectivity is primarily indicated by the presence of HBV DNA in the blood, particularly in the context of viral replication. The virus is transmitted through contact with infected bodily fluids, such as blood, semen, and other sexual fluids. **HBV DNA** is a marker of viral replication, and its presence in the blood indicates that the individual is infectious. The virus replicates in the liver, and HBV DNA is shed into the bloodstream, making it a reliable indicator of infectivity. Enzymes like **hepatitis B virus polymerase** play a crucial role in viral replication, and the presence of HBV DNA is a reflection of this process.

**Why Each Wrong Option is Incorrect**
**Option A:** HBsAg (Hepatitis B surface antigen) is a marker of HBV infection, but it does not directly indicate infectivity. While HBsAg is present in the blood of infected individuals, it does not necessarily reflect the level of viral replication or the presence of infectious virus particles.
**Option B:** Anti-HBc (Antibody to hepatitis B core antigen) is a marker of past or present HBV infection, but it does not indicate infectivity. Anti-HBc can be present in individuals who have cleared the virus or are in the process of recovering from infection.
**Option C:** Anti-HBs (Antibody to hepatitis B surface antigen) is a marker of immunity to HBV, either through vaccination or past infection. Anti-HBs does not indicate infectivity, as it is a protective response to the virus.

**Clinical Pearl / High-Yield Fact**
When assessing HBV infectivity, it is essential to consider the presence of HBV DNA and the patient's viral load. A high viral load is associated with increased infectivity, and treatment strategies should focus on reducing viral replication and shedding.

**Correct Answer: C. HBsAg**