**Core Concept**
The serologic pattern of negative HBsAg and positive anti-HBs (antibody to surface antigen) indicates immunity to hepatitis B virus, typically due to past exposure and successful clearance of the virus.

**Why the Correct Answer is Right**
In this pattern, the absence of HBsAg means the virus is not currently present, and the presence of anti-HBs indicates the immune system has developed protective immunity. This is most consistent with a previous infection that has resolved, leading to the formation of neutralizing antibodies. This pattern is characteristic of **past infection with spontaneous recovery**, not active disease. Chronic hepatitis or acute infection would show different serologic profiles (e.g., HBsAg positive in chronic or acute cases).

**Why Each Wrong Option is Incorrect**
Option B: Chronic active hepatitis requires persistent HBsAg positivity and often elevated liver enzymes, which is not present here.
Option C: Acute hepatitis B infection is marked by HBsAg positivity and often anti-HBs appearing later, but not in the absence of HBsAg.
Option D: "Poor prognosis" is a clinical judgment not supported by serology and is not a condition that fits this pattern.

**Clinical Pearl / High-Yield Fact**
Remember: **Negative HBsAg + Positive anti-HBs = Immunity from past infection**. This is a hallmark of resolved hepatitis B infection and does not imply current disease or risk.

✓ Correct Answer: A. previous hepatitis B infection