**Core Concept:** Human Immunodeficiency Virus (HIV) is a lentivirus causing acquired immune deficiency syndrome (AIDS). Vertical transmission refers to the transmission of HIV from mother to child during pregnancy, childbirth, or breastfeeding. Acquired immune deficiency syndrome is a syndrome characterized by a weakened immune system, making the body susceptible to opportunistic infections and cancers.

**Why the Correct Answer is Right:** Option D is the correct answer because it is not true that administering zidovudine (AZT) to the mother reduces vertical transmission. Zidovudine is a nucleoside analog reverse transcriptase inhibitor, which slows down the progression of HIV but does not completely eliminate the virus. Although it may reduce the risk of vertical transmission, it does not guarantee complete prevention, making it an incorrect option.

**Why Each Wrong Option is Incorrect:**

A. **Option A:** In reality, vertical transmission of HIV from mother to child during childbirth is possible. However, it is not a universal occurrence. Factors influencing transmission include the stage of infection, the mother's viral load, and the baby's immune system.

B. **Option B:** Antiretroviral therapy (ART) is commonly used to prevent vertical transmission, not just AZT. ART regimens include multiple drugs, targeting different stages of the HIV life cycle, which can significantly reduce the risk of vertical transmission.

C. **Option C:** Although breastfeeding can contribute to HIV transmission, the statement "even if the mother is on ART and the baby is on prophylaxis" is incorrect. ART helps in reducing the risk of vertical transmission but does not guarantee complete prevention, making it an incorrect option. Additionally, prophylaxis is typically applied to the baby, not the mother.

**Clinical Pearl:** To prevent vertical HIV transmission, a combination of antiretroviral drugs is administered to both the mother and the newborn during labor, childbirth, and breastfeeding, if applicable. This comprehensive approach reduces the risk significantly but does not ensure complete prevention.