**Core Concept**
Risk of vertical transmission of HIV refers to the transmission of HIV from mother to child during pregnancy, childbirth, or breastfeeding. This transmission occurs due to the presence of HIV in the maternal blood and vaginal secretions, which can infect the fetus or infant during delivery or through breastfeeding.

**Why the Correct Answer is Right**
The risk of vertical transmission of HIV without intervention is approximately 15-20% if the mother is not on antiretroviral therapy (ART) during pregnancy and childbirth. However, this risk is significantly reduced to less than 2% if the mother is on effective ART and the infant receives prophylactic antiretroviral medication after birth. The risk of transmission through breastfeeding is estimated to be around 5-10% if the mother is not on ART, but this risk is eliminated if the infant is formula-fed or the mother is on ART.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not provide a specific percentage or context for the risk of vertical transmission of HIV.
**Option B:** This option is incorrect because it is an incomplete answer and does not take into account the effect of antiretroviral therapy on the risk of transmission.
**Option C:** This option is incorrect because it is an overestimation of the risk of vertical transmission of HIV without intervention.

**Clinical Pearl / High-Yield Fact**
A key factor in reducing the risk of vertical transmission of HIV is the initiation of antiretroviral therapy in the mother as early as possible during pregnancy, ideally before 28 weeks of gestation.

**Correct Answer:** 5-20%