**Core Concept**
The progression of disease in AIDS is closely linked to the depletion of **CD4+ T cells**, which are crucial for the immune system's response to infections. As the **CD4+ T cell count** decreases, the body becomes more susceptible to opportunistic infections and the manifestations of AIDS become more severe. This is due to the impaired immune function.

**Why the Correct Answer is Right**
When the **CD4+ T cell count** falls below a certain threshold, the immune system is severely compromised, leading to the rapid progression of disease and the full-blown manifestation of AIDS. This threshold is typically considered to be around **200 cells/μL**, below which the risk of opportunistic infections and other complications increases significantly. The **immune system dysfunction** at this stage is characterized by the inability to fight off pathogens.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a higher **CD4+ T cell count** would not be associated with the rapid progression of disease.
**Option B:** Similarly, this option is incorrect as it does not accurately reflect the threshold for severe immune compromise.
**Option D:** This option is also incorrect as it does not correspond to the established threshold for the development of severe immune dysfunction in AIDS.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that a **CD4+ T cell count** below **200 cells/μL** is a critical threshold for the diagnosis of AIDS, and patients with counts below this level are at increased risk for opportunistic infections and other complications.

**Correct Answer:** C. 200 cells/μL