**Core Concept**
Mother-to-child transmission (MTCT) of HIV is a significant mode of HIV transmission, particularly in resource-limited settings. MTCT occurs when HIV is transmitted from an infected mother to her child during pregnancy, childbirth, or breastfeeding. This mode of transmission is a major public health concern, as it can lead to significant childhood morbidity and mortality.

**Why the Correct Answer is Right**
The correct answer, 25%, reflects the estimated risk of MTCT of HIV in the absence of effective interventions. This risk is attributed to the presence of HIV in the maternal blood, vaginal secretions, and breast milk. The virus can infect the fetus during pregnancy, particularly during the third trimester, or during labor and delivery through mucosal exposure. Breastfeeding is another critical period for MTCT, as HIV can be transmitted through breast milk.

The risk of MTCT can be significantly reduced with the use of antiretroviral therapy (ART) in the mother, elective cesarean delivery, and avoidance of breastfeeding. In settings where these interventions are available, the risk of MTCT can be reduced to less than 2%.

**Why Each Wrong Option is Incorrect**
**Option B:** 50% - This option overestimates the risk of MTCT in the absence of effective interventions. While the risk is significant, it is not as high as 50%.

**Option C:** 60% - Similar to Option B, this option overestimates the risk of MTCT. The actual risk is lower than 60%.

**Option D:** 75% - This option is an extreme overestimation of the risk of MTCT. The actual risk is significantly lower than 75%.

**Clinical Pearl / High-Yield Fact**
A key strategy to reduce the risk of MTCT is the use of antiretroviral therapy (ART) in the mother. ART has been shown to reduce the risk of MTCT by up to 70%. Early initiation of ART, ideally before 28 weeks of gestation, is critical to achieving this reduction in risk.

**✓ Correct Answer: A. 25%**