**Core Concept**
Sharing needles or syringes contaminated with blood is a significant risk factor for HIV transmission due to the presence of high concentrations of the virus in the blood. This practice facilitates the exchange of bodily fluids, increasing the likelihood of viral transmission.

**Why the Correct Answer is Right**
The risk of HIV transmission via needle sharing is extremely high, with a reported transmission rate of up to 30% per shared needle. This is because the virus is present in the blood at high concentrations, and the act of sharing needles involves direct contact with potentially infected bodily fluids. The viral load in the blood of individuals with untreated HIV can reach up to 10^7 copies/mL, making it a highly infectious medium.

**Why Each Wrong Option is Incorrect**
**Option A:** Sexual transmission of HIV is a significant risk factor, but the transmission rate is generally lower than that associated with needle sharing. According to the CDC, the risk of HIV transmission via vaginal sex is approximately 1 in 1,000 per act.

**Option B:** Mother-to-child transmission of HIV during pregnancy, childbirth, or breastfeeding is a concern, but the risk can be significantly reduced with antiretroviral therapy. The transmission rate can be as low as 1-2% with proper treatment.

**Option C:** Blood transfusion from an HIV-positive donor is a rare but serious risk in countries with inadequate blood screening. However, with modern screening techniques, the risk of transmission via blood transfusion is extremely low in developed countries.

**Clinical Pearl / High-Yield Fact**
It's essential to note that needle sharing is a significant risk factor for not only HIV transmission but also other bloodborne pathogens, such as hepatitis B and C.

**Correct Answer: D. Needle sharing**