**Core Concept**
The most efficient route of HIV transmission refers to the mode of transmission that results in the highest viral load and highest risk of infection. This concept is crucial in understanding the prevention and control of HIV/AIDS.

**Why the Correct Answer is Right**
The most efficient route of HIV transmission is through **unprotected vaginal or anal intercourse**. This is because the mucosal surfaces of the vagina and anus provide a direct route for the virus to enter the bloodstream, and the absence of a condom or other barrier increases the viral load and the risk of transmission. The high concentration of CD4+ T cells in the genital tract also facilitates the replication of HIV, making this route particularly efficient.

**Why Each Wrong Option is Incorrect**
**Option A:** Blood transfusion is a mode of HIV transmission, but it is not the most efficient route. This is because the risk of transmission through blood transfusion is relatively low, and modern blood screening techniques have significantly reduced the risk.

**Option B:** Mother-to-child transmission is a significant mode of HIV transmission, but it is not the most efficient route. This is because the risk of transmission can be significantly reduced with antiretroviral therapy and other preventive measures.

**Option C:** Needlestick injury is a mode of HIV transmission, but it is not the most efficient route. This is because the risk of transmission through needlestick injury is relatively low, and the use of personal protective equipment and proper needle disposal techniques can minimize the risk.

**Clinical Pearl / High-Yield Fact**
The most efficient route of HIV transmission is through unprotected vaginal or anal intercourse, highlighting the importance of condom use and other barrier methods in preventing HIV transmission.

**Correct Answer: C. Needlestick injury is a mode of HIV transmission, but it is not the most efficient route. This is because the risk of transmission through needlestick injury is relatively low, and the use of personal protective equipment and proper needle disposal techniques can minimize the risk.