**Core Concept**
The question is testing the management of vertical transmission of hepatitis B from a mother to her newborn. Vertical transmission of hepatitis B occurs when the virus is transmitted from mother to child during pregnancy, childbirth, or breastfeeding. The goal of post-exposure prophylaxis is to prevent this transmission.

**Why the Correct Answer is Right**
The standard treatment for an infant born to an infected hepatitis B mother is hepatitis B immune globulin (HBIG) and the first dose of hepatitis B vaccine. HBIG provides passive immunity to the infant, while the vaccine starts to stimulate active immunity. This combination is crucial in preventing hepatitis B virus (HBV) infection in the newborn. The Centers for Disease Control and Prevention (CDC) and World Health Organization (WHO) recommend this approach for all infants born to hepatitis B-infected mothers.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because hepatitis B surface antigen (HBsAg) testing is not a treatment for vertical transmission.
**Option B:** This option is incorrect because antiviral therapy is not the standard treatment for an infant born to an infected hepatitis B mother.
**Option C:** This option is incorrect because liver transplantation is not a treatment for vertical transmission of hepatitis B.

**Clinical Pearl / High-Yield Fact**
Infants born to hepatitis B-infected mothers should receive hepatitis B immune globulin (HBIG) and the first dose of hepatitis B vaccine within 12 hours of birth to prevent vertical transmission.

**Correct Answer: D. HBIG and hepatitis B vaccine**