**Core Concept:** Transplacental transmission of Acquired Immunodeficiency Syndrome (AIDS) occurs when the virus, Human Immunodeficiency Virus (HIV), enters the fetal circulation through the placenta, leading to infection in the fetus. HIV primarily targets CD4+ T cells, which are crucial for immune system function.

**Why the Correct Answer is Right:** AIDS is primarily transmitted through contact with infected body fluids, such as blood, semen, vaginal secretions, breast milk, and from an infected mother to a fetus during pregnancy, childbirth, or breastfeeding. In the case of transplacental transmission, the virus enters the fetal circulation via the placenta, leading to infection in the fetus. This mode of transmission is more common in developing countries due to limited access to antenatal care and antiretroviral therapy. Antiretroviral therapy (ART) can significantly reduce the risk of transplacental transmission.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because AIDS primarily spreads through contact with infected body fluids, not through breastfeeding alone.
B. Although breastfeeding is a mode of transmission, the correct answer includes all modes of transplacental transmission.
C. This option is incorrect as AIDS can be transmitted through multiple routes, not just through a single event (sexual intercourse).
D. This option is incorrect because AIDS primarily spreads through contact with infected body fluids, not by direct contact.

**Clinical Pearl:**

In low-resource settings, routine screening of HIV in pregnant women is essential to identify infected individuals and provide them with ART to minimize the risk of transplacental transmission. In developed countries, HIV testing and counseling during pregnancy have significantly reduced the number of transmissions. Prompt initiation of ART during pregnancy and delivery can further decrease the risk of transmission.