**Core Concept**
HIV/AIDS is primarily a disease of the immune system caused by the human immunodeficiency virus (HIV), which targets and destroys CD4 T lymphocytes, leading to immunocompromised states and increased susceptibility to opportunistic infections.

**Why the Correct Answer is Right**
The correct answer is related to the direct effect of HIV infection on tissues, specifically the lymphoid tissues where CD4 T cells reside. HIV infection directly causes the destruction of these cells through viral replication and apoptosis, leading to a decline in CD4 counts and compromised immune function. This results in the patient's increased susceptibility to opportunistic infections and malignancies.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not directly related to the effect of HIV on tissues. While HIV does affect the nervous system, it is not the primary mechanism by which the virus causes disease.
**Option B:** This option is incorrect because HIV primarily targets CD4 T cells, not macrophages. While macrophages can be infected by HIV, they are not the primary target of the virus.
**Option C:** This option is incorrect because HIV does not primarily affect the gastrointestinal tract through direct infection of tissues. While HIV can cause gastrointestinal symptoms, they are more related to opportunistic infections and malabsorption.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that HIV/AIDS is a disease of immune dysfunction, and the primary target of the virus is the CD4 T cell. This understanding is crucial for diagnosing and managing HIV/AIDS patients.

**Correct Answer:** B.