**Core Concept**
HIV (Human Immunodeficiency Virus) is a lentivirus that primarily targets and infects CD4+ T lymphocytes, leading to a gradual decline in cell-mediated immunity and ultimately, the progression to AIDS (Acquired Immunodeficiency Syndrome). This selective targeting of CD4+ cells is a hallmark of HIV infection.

**Why the Correct Answer is Right**
HIV uses its envelope glycoprotein, gp120, to bind to the CD4 receptor on the surface of T cells, facilitating viral entry and replication. This interaction between gp120 and CD4 triggers a conformational change in the gp41 subunit, leading to the fusion of the viral envelope with the host cell membrane. The subsequent release of viral nucleic acid into the host cell allows for the expression of viral genes and the production of new viral particles. The selective targeting of CD4+ cells by HIV results in a gradual decline in the number of these cells, ultimately leading to immunocompromise and increased susceptibility to opportunistic infections.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as HIV can be found in various cell types, including macrophages and dendritic cells, in addition to CD4+ T lymphocytes.
**Option B:** Incorrect, as HIV can be isolated from saliva, semen, vaginal fluids, and breast milk, among other bodily fluids.
**Option C:** Incorrect, as HIV can be transmitted through blood transfusions, contaminated needles, and mother-to-child transmission during pregnancy or childbirth.

**Clinical Pearl / High-Yield Fact**
HIV infection can be diagnosed using various laboratory tests, including the Western blot, ELISA, and PCR (Polymerase Chain Reaction) assays. These tests detect the presence of HIV antibodies, antigens, or viral RNA in the blood.

**Correct Answer:** D