## Core Concept
The presence of HBsAg (Hepatitis B surface antigen) indicates an active Hepatitis B infection. The absence of anti-HBs (antibody to Hepatitis B surface antigen) suggests that the person has not developed immunity to the virus, either through vaccination or past infection. This scenario can occur in acute or chronic Hepatitis B infection.

## Why the Correct Answer is Right
In a person who is HBsAg positive but anti-HBs negative, the next step would involve assessing the individual's Hepatitis B status further. This includes evaluating for the presence of other Hepatitis B markers such as HBeAg (Hepatitis B e-antigen), anti-HBe (antibody to Hepatitis B e-antigen), and HBV DNA levels to determine the viral activity and infectivity. The correct approach would be to **check for IgM anti-HBc (Immunoglobulin M antibody to Hepatitis B core antigen)** to differentiate between acute and chronic infection. IgM anti-HBc positivity indicates acute Hepatitis B infection, while IgG anti-HBc positivity suggests a chronic infection.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, but typically, an incorrect next step might involve not further evaluating the Hepatitis B status or misinterpreting the markers.
- **Option B:** Similarly, without specifics, an incorrect approach could involve immediate antiviral therapy without assessing the full spectrum of Hepatitis B markers.
- **Option C:** This could potentially be an incorrect approach if it involves a wrong interpretation of the serological markers or not considering the full clinical picture.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the presence of HBsAg and the absence of anti-HBs indicate an active infection. The differentiation between acute and chronic infection is crucial for management and is done by checking for **IgM anti-HBc**. This clinical scenario highlights the importance of understanding Hepatitis B serology for proper patient management.

## Correct Answer: D. Check for IgM anti-HBc.