**Core Concept**
Kaposi's sarcoma (KS) is a type of skin cancer that predominantly affects individuals with compromised immune systems, particularly those with advanced HIV/AIDS. The pathogenesis of KS is closely linked to the human herpesvirus 8 (HHV-8), which infects and transforms endothelial cells. The proliferation of these transformed cells leads to the characteristic lesions of KS.

**Why the Correct Answer is Right**
The development of KS is strongly associated with a low CD4+ lymphocyte count, typically below 200 cells/μL. This is because the immune system's ability to control HHV-8 infection and prevent the proliferation of transformed endothelial cells is severely impaired in individuals with advanced HIV/AIDS. The CD4+ lymphocytes play a crucial role in orchestrating the immune response against viral infections, and their depletion allows HHV-8 to persist and cause disease. The transformed endothelial cells in KS lesions express various markers, including vascular endothelial growth factor (VEGF), which promotes angiogenesis and tumor growth.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because KS can occur at any CD4+ lymphocyte count, including in individuals with higher counts.
**Option B:** This option is incorrect because KS is not exclusively associated with a specific CD4+ lymphocyte count; rather, it is a marker of advanced HIV/AIDS and immune system compromise.
**Option C:** This option is incorrect because KS is not typically seen in individuals with normal CD4+ lymphocyte counts.

**Clinical Pearl / High-Yield Fact**
In patients with HIV/AIDS, a CD4+ lymphocyte count below 200 cells/μL is a significant risk factor for developing opportunistic infections, including KS, as well as other cancers such as non-Hodgkin lymphoma.

**Correct Answer:** A. Below 200 cells/μL.