**Core Concept**
The transmission of HIV from mother to child occurs through various routes, but the most common cause in infants is vertical transmission, also known as mother-to-child transmission. This can happen during pregnancy, childbirth, or breastfeeding.

**Why the Correct Answer is Right**
The most common cause of HIV infection in infants is through vertical transmission, specifically during pregnancy and childbirth. This occurs when the virus is passed from the mother's blood to the infant's bloodstream, often through the placenta during fetal development. The risk of transmission is higher if the mother has a high viral load, is not on antiretroviral therapy, or has a history of genital ulcers or other sexually transmitted infections.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while breastfeeding can be a route of transmission, it is not the most common cause of HIV infection in infants.
**Option B:** This option is incorrect because HIV infection in infants is not primarily caused by horizontal transmission, such as through blood transfusions or needle sharing.
**Option C:** This option is incorrect because vertical transmission during pregnancy and childbirth is a more common cause of HIV infection in infants than transmission through sexual abuse or assault.

**Clinical Pearl / High-Yield Fact**
Infants born to HIV-positive mothers are at risk of vertical transmission, and antiretroviral therapy given to the mother and infant can significantly reduce this risk. It is essential for healthcare providers to screen all pregnant women for HIV and provide appropriate treatment to prevent mother-to-child transmission.

**Correct Answer: D. Vertical transmission during pregnancy and childbirth is the most common cause of HIV infection in infants.**