**Core Concept**
The question is testing the student's knowledge of antiviral medications used to treat viral hepatitis and HIV. The underlying principle is the pharmacological action of antiviral drugs, specifically those that target viral replication.

**Why the Correct Answer is Right**
Entecavir is a nucleoside analog that inhibits viral replication by interfering with the viral DNA polymerase. It is used to treat both HIV and Hepatitis B infections. Entecavir is a potent inhibitor of the HBV DNA polymerase, leading to a decrease in viral load and a reduction in liver inflammation.

**Why Each Wrong Option is Incorrect**
**Option A:** Tenofovir is another antiviral medication used to treat HIV, but it is not typically used to treat Hepatitis B. While it has some activity against HBV, it is not the preferred treatment for this indication.

**Option B:** Emtricitabine is an antiretroviral medication used to treat HIV, but it is not typically used to treat Hepatitis B. It is often used in combination with other medications to treat HIV, but it is not a first-line treatment for HBV.

**Option C:** Lamivudine is an antiretroviral medication used to treat both HIV and Hepatitis B. However, it is not the preferred treatment for HBV due to the risk of resistance.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the treatment of viral hepatitis B often involves a combination of antiviral medications, and the choice of medication depends on the individual patient's needs and medical history.

**Correct Answer:** C. Lamivudine is an antiretroviral medication used to treat both HIV and Hepatitis B.