**Question:** Least common mode of transmission of HIV -

A. Sexual contact
B. Transfusion of infected blood
C. Ingestion of infected food or water
D. Injection drug use

**Core Concept:** Human Immunodeficiency Virus (HIV) is primarily transmitted through specific modes involving the exposure of the body to infected bodily fluids. The least common modes of transmission are those involving non-sexual routes.

**Why the Correct Answer is Right:**

The correct answer, **C. Ingestion of infected food or water**, is the least common mode of HIV transmission. While HIV can be transmitted through contaminated food and water, this is extremely rare due to the nature of the virus and the necessary conditions for transmission. The virus primarily requires direct exposure to the blood, semen, vaginal fluids, breast milk, or ocular fluids to be transmitted. Ingestion of HIV-infected food or water is extremely unlikely unless the food or water is contaminated with infected bodily fluids, a scenario that is highly improbable in modern societies.

**Why Each Wrong Option is Incorrect:**

A. **Sexual contact (A)** - This is the most common mode of HIV transmission, involving direct contact with infected blood, semen, vaginal fluids, breast milk, or ocular fluids.

B. **Transfusion of infected blood (B)** - While rare, HIV can be transmitted through blood transfusions if the donor is infected. However, this is less common than sexual transmission.

D. **Injection drug use (D)** - HIV can be transmitted through sharing contaminated needles or syringes, but this mode of transmission is still more common than ingestion of contaminated food or water.

**Clinical Pearl:** The least common mode of HIV transmission is ingesting contaminated food or water. However, it is essential to remember that HIV primarily spreads through direct contact with infected bodily fluids. This knowledge helps prevent the spread of the virus and provides context for discussing transmission pathways during patient education.