## **Core Concept**
Mycobacterium avium complex (MAC) infections are a type of opportunistic infection that primarily affects individuals with compromised immune systems, particularly those with advanced HIV infection. The risk of MAC infection is closely related to the level of immunosuppression, as measured by CD4+ T-cell counts.

## **Why the Correct Answer is Right**
The correct answer, , indicates that MAC infections are common when the CD4+ count falls below 50 cells/mm^3. This is because a CD4+ count below 50 cells/mm^3 signifies severe immunosuppression, where the body's ability to fight off opportunistic pathogens like MAC is significantly impaired. MAC is an environmental organism that can cause disseminated infection in individuals with such low CD4+ counts, affecting multiple organ systems.

## **Why Each Wrong Option is Incorrect**
- **Option A:** . This option suggests that MAC infections are common at higher CD4+ counts, which is not accurate. While some opportunistic infections occur at higher CD4+ counts, MAC infections typically occur at much lower counts.
- **Option B:** . This option implies that MAC infections are common at a slightly higher CD4+ count than the correct answer but still indicates a level of immunosuppression. However, it is less specific and not as strongly associated with the risk of MAC infection as the correct answer.
- **Option D:** . This option suggests an even lower threshold, but it is less commonly cited as the specific threshold for increased risk of MAC infection compared to .

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that prophylaxis against MAC is often recommended for patients with HIV/AIDS and CD4+ counts below 50 cells/mm^3. This typically involves the use of azithromycin or rifabutin, among other regimens, to prevent the onset of MAC infection.

## **Correct Answer:** . 50 cells/mm^3.