In a patient, only Anti HBsAg is positive in serum. All other viral markers are negative. This indicates:
**Core Concept**
The presence of Anti-HBsAg (antibody to hepatitis B surface antigen) in the absence of other viral markers suggests a specific stage of hepatitis B infection or immunity. This indicates that the patient has developed immunity against the hepatitis B virus, which can occur through either vaccination or natural infection and subsequent recovery.
**Why the Correct Answer is Right**
The presence of Anti-HBsAg is a marker of immunity to hepatitis B. In this scenario, the patient has developed antibodies against the surface antigen of the virus, which is the first step in the immune response to hepatitis B. This indicates that the patient has either been vaccinated against hepatitis B or has recovered from a past infection. The absence of other viral markers such as HBsAg (hepatitis B surface antigen), HBeAg (hepatitis B e-antigen), and anti-HBe (antibody to hepatitis B e-antigen) supports this interpretation.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the presence of Anti-HBsAg alone does not necessarily indicate an active or chronic infection. The absence of other viral markers provides further evidence that the patient is not currently infected.
**Why Each Wrong Option is Incorrect (continued)**
**Option B:** This option is incorrect because it implies that the patient is currently infected with hepatitis B. However, the presence of Anti-HBsAg and the absence of HBsAg, HBeAg, and anti-HBe suggest that the patient has developed immunity against the virus.
**Why Each Wrong Option is Incorrect (continued)**
**Option C:** This option is incorrect because it is a stage of chronic hepatitis B infection characterized by the presence of HBsAg and anti-HBe. The patient in this scenario does not have HBsAg, making this option incorrect.
**Why Each Wrong Option is Incorrect (continued)**
**Option D:** This option is incorrect because it implies that the patient has a history of hepatitis B infection, but is currently infected. However, the presence of Anti-HBsAg and the absence of HBsAg, HBeAg, and anti-HBe suggest that the patient has developed immunity against the virus and is not currently infected.
**Clinical Pearl / High-Yield Fact**
It is essential to note that the presence of Anti-HBsAg alone can be a result of either vaccination or natural infection and subsequent recovery. Therefore, it is crucial to consider the patient's vaccination history and past medical history when interpreting the results.
**Correct Answer: D. Recovered from a past infection.**