A 40-year-old man complains of having yellow skin and sclerae, abdominal tenderness, and dark urine. Physical examination reveals jaundice and mild hepatomegaly Laboratory studies demonstrate elevated serum bilirubin (3.1 mg/dL), decreased serum albumin (2.5 g/dL), and prolonged prothrombin time (17 seconds). Serologic tests reveal antibodies to hepatitis B core antigen (IgG anti-HBcAg). The serum is also positive for HBsAg and HBeAg. What glycoprotein on virally infected hepatocytes provides a target for cell-mediated cytotoxicity in the patient?

## **Core Concept**
The patient's symptoms and laboratory findings are indicative of acute hepatitis B infection. The presence of HBsAg (hepatitis B surface antigen), HBeAg (hepatitis B e-antigen), and IgG anti-HBcAg (antibody to hepatitis B core antigen) confirms the diagnosis. In hepatitis B infection, cell-mediated cytotoxicity plays a crucial role in liver injury.

## **Why the Correct Answer is Right**
The correct answer involves understanding the immune response to hepatitis B virus (HBV) infection. In HBV infection, **HBsAg** (hepatitis B surface antigen) is expressed on the surface of infected hepatocytes. The immune system, specifically cytotoxic T lymphocytes (CTLs), recognizes and targets infected hepatocytes displaying viral antigens on their surface through major histocompatibility complex (MHC) class I molecules. HBsAg on the surface of infected hepatocytes serves as a target for cell-mediated cytotoxicity.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not specify a known target for cell-mediated immunity in the context of HBV infection.
- **Option B:** This option is incorrect as it does not accurately represent a glycoprotein target on virally infected hepatocytes for cell-mediated cytotoxicity in HBV infection.
- **Option C:** This option might seem related but is not the correct target for cell-mediated cytotoxicity in the context provided.
- **Option D:** This option is incorrect because, although important in the HBV life cycle and immune response, it is not the glycoprotein specifically targeted by cell-mediated immunity on infected hepatocytes.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that HBsAg expression on hepatocytes makes them susceptible to immune-mediated destruction. This process is critical in the pathogenesis of liver injury in acute and chronic HBV infections. Understanding the role of HBsAg in immune targeting helps in managing and preventing the progression of HBV infection.

## **Correct Answer:** . HBsAg

✓ Correct Answer: C. Class I HLA molecules