## **Core Concept**
The question tests knowledge of viral infections and their potential to cross the placenta, which is a critical concept in obstetrics and virology. The ability of viruses to cross the placenta can significantly impact fetal health and pregnancy outcomes. Various viruses have different propensities for transplacental transmission.

## **Why the Correct Answer is Right**
The Hepatitis B virus (HBV) primarily infects hepatocytes and does not typically cross the placenta to infect the fetus. While there is a risk of mother-to-child transmission of HBV during delivery, this is more related to exposure to infected maternal blood and vaginal secretions rather than transplacental transmission. The risk of vertical transmission of HBV can be significantly reduced with proper antenatal screening and administration of hepatitis B immune globulin and hepatitis B vaccine to the newborn.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Rubella virus can cross the placenta, especially if the infection occurs during the first trimester, leading to congenital rubella syndrome.
- **Option B:** Cytomegalovirus (CMV) is known to cross the placenta and is a leading cause of congenital viral infections worldwide.
- **Option D:** Varicella-zoster virus can also cross the placenta, although the risk is higher if the maternal infection occurs during the second half of pregnancy.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that while many viral infections can be managed during pregnancy to prevent fetal transmission, screening for certain viruses (like HBV, HIV, and CMV in some cases) is crucial for preventing mother-to-child transmission. Understanding which viruses are more likely to cross the placenta helps in the management and counseling of pregnant women.

## **Correct Answer:** D. Hepatitis B.