**Core Concept**
The risk of HIV transmission through various routes is influenced by several factors, including viral load, mucosal integrity, and the presence of other sexually transmitted infections (STIs). Understanding the relative risks associated with different modes of HIV transmission is crucial for targeted prevention and intervention strategies.

**Why the Correct Answer is Right**
Heterosexual intercourse carries the least chance of transmitting HIV infection compared to other options. This is because the risk of HIV transmission through vaginal or anal intercourse is significantly lower than through other modes of transmission, such as blood transfusion, vertical transmission, or IV drug abuse. The mucosal integrity of the vagina and penis, combined with the presence of protective immune cells in the genital tract, reduces the likelihood of HIV transmission. However, this does not mean that heterosexual intercourse is completely risk-free, and consistent condom use and other preventive measures are still essential to minimize the risk of HIV transmission.

**Why Each Wrong Option is Incorrect**
**Option B:** Blood transfusion carries a significant risk of HIV transmission, particularly if the blood is not properly screened or if the screening tests are not sensitive enough. This is because HIV can be present in the blood of infected individuals, even if they do not show symptoms.
**Option C:** Vertical transmission, also known as mother-to-child transmission, occurs when HIV is transmitted from an infected mother to her child during pregnancy, childbirth, or breastfeeding. This mode of transmission is a significant concern and can be prevented with antiretroviral therapy and other interventions.
**Option D:** IV drug abusers are at high risk of HIV transmission through sharing needles or other equipment contaminated with HIV-infected blood. This mode of transmission is a significant public health concern, particularly among individuals who inject drugs.

**Clinical Pearl / High-Yield Fact**
Consistent condom use can reduce the risk of HIV transmission through heterosexual intercourse by 80-90%. However, condoms are not foolproof, and other preventive measures, such as antiretroviral therapy and pre-exposure prophylaxis (PrEP), may be necessary to minimize the risk of HIV transmission.

**✓ Correct Answer: A. Heterosexual Intercourse**