**Core Concept**
The CCR5 protein is a chemokine receptor involved in the immune response, specifically in the trafficking of immune cells. Mutations in the CCR5 gene can lead to altered function or expression of the CCR5 protein, affecting its role in HIV entry into host cells.

**Why the Correct Answer is Right**
The mutation of CCR5 protein is associated with high resistance to HIV infection because it disrupts the interaction between the virus and the host cell. The CCR5 protein serves as a co-receptor for HIV entry into CD4+ T cells. Certain mutations, such as the CCR5-Δ32 deletion, lead to a non-functional CCR5 protein, which prevents HIV from using this receptor for entry. As a result, individuals with this mutation are more resistant to HIV infection. This is because the virus cannot use the CCR5 receptor to gain entry into host cells, thereby reducing the risk of infection.

**Why Each Wrong Option is Incorrect**
**Option B:** This option is incorrect because the mutation of CCR5 protein does not increase susceptibility to HIV infection. Instead, it reduces the risk of infection by preventing the virus from entering host cells.

**Option C:** This option is incorrect because the mutation of CCR5 protein does not cause resistance to anti-retroviral therapy. While the mutation can reduce the risk of HIV infection, it does not affect the efficacy of anti-retroviral medications.

**Option D:** This option is incorrect because the mutation of CCR5 protein does not increase the virulence of the virus. Virulence refers to the ability of a pathogen to cause disease, and the mutation of CCR5 protein does not alter this property of HIV.

**Clinical Pearl / High-Yield Fact**
The CCR5-Δ32 deletion is a well-documented example of how genetic variation can influence susceptibility to infectious diseases. This deletion is found in approximately 10-15% of people of European descent and provides a natural barrier against HIV infection.

**✓ Correct Answer: A. High resistance to HIV infection**