**Core Concept**
HBV (Hepatitis B Virus) infection is a significant public health concern, particularly among individuals who engage in high-risk behaviors such as IV-drug use. After exposure to HBV, the body's immune response is triggered, leading to the production of various viral markers that can be detected through serological tests. Understanding the sequence of these markers is crucial for diagnosing and managing HBV infection.

**Why the Correct Answer is Right**
The first viral marker detected after HBV infection is the **HBsAg** (Hepatitis B surface antigen). This occurs due to the immediate production of HBsAg by infected hepatocytes. HBsAg is a protein on the surface of the HBV virus and is the first indicator of HBV infection. It can be detected as early as 1-2 weeks after exposure and remains present until the virus is cleared or the individual becomes chronically infected.

**Why Each Wrong Option is Incorrect**
**Option A:** **HBeAg** (Hepatitis B e-antigen) is a marker of viral replication and is usually detected after HBsAg. Its presence indicates that the individual is highly infectious.

**Option B:** **Anti-HBs** (Antibody to Hepatitis B surface antigen) is an antibody produced in response to HBsAg and is usually detected after HBV infection has been cleared or the individual has been vaccinated. Its presence indicates immunity to HBV.

**Option C:** **Anti-HBc** (Antibody to Hepatitis B core antigen) is an antibody produced in response to the core antigen of HBV. It can be detected in both acute and chronic HBV infections, but it is not the first marker detected after exposure.

**Clinical Pearl / High-Yield Fact**
In the context of HBV infection, it's essential to remember that HBsAg is the first marker detected after exposure, and its presence is a key indicator of HBV infection. This knowledge is crucial for public health interventions, particularly among high-risk populations.

**Correct Answer:** A. HBsAg