**Core Concept**
Acute hepatitis B infection is characterized by the presence of viral replication and liver inflammation. DNA polymerase is an enzyme essential for viral DNA replication, and its presence indicates active viral replication.

**Why the Correct Answer is Right**
The correct answer is A. DNA polymerase because it is a marker of active viral replication, which is a hallmark of acute hepatitis B infection. Hepatitis B virus (HBV) contains a reverse transcriptase enzyme, also known as DNA polymerase, which is responsible for converting the viral RNA into DNA. The presence of DNA polymerase in the serum indicates that the virus is actively replicating, which is a characteristic feature of acute hepatitis B infection. This enzyme is not present in the serum during the inactive or chronic phases of the infection.

**Why Each Wrong Option is Incorrect**
**Option B:** Hepatitis core antigen is a marker of chronic hepatitis B infection, not acute infection. It is present in the serum during the chronic phase of the infection and is not a reliable marker for acute infection.

**Option C:** Anti-HBs (antibody to hepatitis B surface antigen) is a marker of immunity to hepatitis B, either from vaccination or past infection. It is not a marker of acute infection.

**Option D:** IgG to core antigen is a marker of chronic hepatitis B infection, indicating a past or ongoing infection. It is not a reliable marker for acute infection.

**Clinical Pearl / High-Yield Fact**
Remember that DNA polymerase is a key enzyme in viral replication, and its presence is a critical marker for diagnosing acute hepatitis B infection.

**Correct Answer:**
✓ Correct Answer: A. DNA polymerase