**Core Concept**
Acute viral hepatitis caused by HBV is a liver infection characterized by inflammation and damage to hepatocytes. The liver's response to this injury involves the production of various markers, including enzymes and proteins, which can be detected in the blood.

**Why the Correct Answer is Right**
The correct answer is **HBsAg** (Hepatitis B surface antigen). HBsAg is a protein on the surface of the HBV virus, and its presence in the blood indicates an active HBV infection. This marker is commonly used as a screening test for HBV infection. When HBV infects a hepatocyte, it replicates and produces new viral particles, including HBsAg. The presence of HBsAg in the blood signifies that the virus is actively replicating and causing liver damage.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because **AST (Aspartate Aminotransferase)** is an enzyme that is elevated in various liver conditions, including hepatitis, but it is not a specific marker for HBV infection.
* **Option B:** This option is incorrect because **ALT (Alanine Aminotransferase)** is another enzyme that is elevated in liver conditions, but it is not a specific marker for HBV infection.
* **Option C:** This option is incorrect because **IgM anti-HBc (Immunoglobulin M antibody to Hepatitis B core antigen)** is an antibody that is produced in response to HBV infection, but it is not the primary marker for acute HBV infection.

**Clinical Pearl / High-Yield Fact**
HBsAg is a critical marker for diagnosing HBV infection, and its presence or absence can guide treatment decisions. It's essential to remember that HBsAg can persist in the blood for months or even years after HBV infection, so its presence does not necessarily indicate an active infection.

**Correct Answer:** D. HBsAg