**Core Concept**
Hepatitis B in children can be caused by vertical transmission (mother-to-child) during birth, horizontal transmission from an infected family member or caregiver, or by other routes such as blood transfusions. The immune response in children is critical in determining the outcome of the infection.

**Why the Correct Answer is Right**
In a 3-year-old child, the most common cause of Hepatitis B is vertical transmission from the mother. This occurs when the mother is positive for Hepatitis B surface antigen (HBsAg) during pregnancy. The risk of transmission is highest if the mother is also positive for Hepatitis B e-antigen (HBeAg) and has high viral loads. **Passive immunity** is not sufficient to prevent infection, and the child can become a chronic carrier of the virus. The child's immune system can clear the virus in some cases, but chronic infection is more likely in children who acquire the virus vertically.

**Why Each Wrong Option is Incorrect**
* **Option A:** Not mentioned in standard references - likely incorrect.
* **Option B:** Not specified in standard references - likely incorrect.
* **Option D:** This is actually a rare cause of hepatitis B in children.

**Clinical Pearl / High-Yield Fact**
Hepatitis B vaccination is a crucial preventive measure, especially for newborns of mothers who are positive for HBsAg. The vaccine can prevent vertical transmission and chronic infection.

**Correct Answer:** C.