In a patient only anti HBs Ag is positive in serum, all other markers are negative. This indicates ?
**Core Concept**
The presence of anti-HBs Ag (antibody to hepatitis B surface antigen) indicates a specific immune response to hepatitis B virus (HBV) surface antigen. This is a crucial marker in the serological diagnosis of HBV infection and its complications.
**Why the Correct Answer is Right**
The presence of anti-HBs Ag alone, without other HBV markers, suggests that the patient has been immunized against HBV, either through vaccination or resolution of a previous HBV infection. The patient's immune system has mounted a specific response to the HBV surface antigen, producing antibodies against it. This is a sign of immunity, indicating that the patient is unlikely to be infectious or develop HBV-related liver disease.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the presence of anti-HBs Ag alone does not indicate acute HBV infection, which is typically characterized by the presence of HBs Ag, HBV DNA, and IgM antibodies against HBV core antigen (IgM anti-HBc).
**Option B:** This option is incorrect because the presence of anti-HBs Ag alone does not indicate chronic HBV infection, which is typically characterized by the presence of HBs Ag, HBV DNA, and IgG antibodies against HBV core antigen (IgG anti-HBc).
**Option C:** This option is incorrect because the presence of anti-HBs Ag alone does not indicate HBV-related liver disease, such as hepatitis or cirrhosis, which is typically characterized by the presence of HBs Ag, HBV DNA, and liver enzyme abnormalities.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the presence of anti-HBs Ag alone in a patient's serum indicates immunity to HBV, either through vaccination or resolution of a previous infection. This is a key distinction from other HBV markers, which can indicate active infection or liver disease.
**Correct Answer: C. Immunity to HBV.**