**Core Concept**
Breast milk transmission refers to the transfer of infectious agents or toxins from a mother to her infant through breast milk. This can occur through various mechanisms, including the presence of pathogens in the milk or the mother's immune system's response to a particular infection.

**Why the Correct Answer is Right**
HIV (Human Immunodeficiency Virus) is a well-documented example of an infectious agent that can be transmitted through breast milk. This occurs when the virus is present in the mother's bodily fluids, including breast milk. The transmission of HIV through breast milk is a significant concern in regions with high HIV prevalence, and guidelines often recommend alternative feeding options for infants born to HIV-positive mothers.

**Why Each Wrong Option is Incorrect**
* **Option A:** While breast milk can transmit infectious agents, it is not a primary route for the transmission of Hepatitis B virus (HBV). HBV transmission typically occurs through perinatal exposure to infected bodily fluids during birth.
* **Option B:** Breast milk is not a significant route for the transmission of Cytomegalovirus (CMV). CMV is a common virus that can be transmitted through saliva, urine, and other bodily fluids, but breast milk transmission is rare.
* **Option C:** Breast milk is not a primary route for the transmission of Rotavirus. Rotavirus is a common cause of diarrhea in infants, and transmission typically occurs through fecal-oral contact.

**Clinical Pearl / High-Yield Fact**
In regions with high HIV prevalence, the World Health Organization recommends that infants born to HIV-positive mothers receive antiretroviral therapy (ART) and be breastfed for the first 6 months, under the condition that the mother's viral load is suppressed.

**Correct Answer: D. HIV. The virus responsible for AIDS.**