American Board of Internal Medicine (ABIM) Certification Practice Exam

Mycobacterium avium complex infection in HIV patients typically occurs when CD4 count is:

1. Less than 100 cells/mm3

2. Less than 50 cells/mm3

3. Less than 200 cells/mm3

4. Less than 350 cells/mm3

The correct answer is: Less than 50 cells/mm3

Mycobacterium avium complex (MAC) infection is particularly associated with advanced immunosuppression in patients with HIV. The risk of developing MAC significantly increases when the CD4 cell count drops below a certain threshold. Specifically, MAC infection is commonly seen in individuals with a CD4 count less than 100 cells/mm3, but it becomes more critically prevalent in those with counts below 50 cells/mm3. This level of immunosuppression indicates a severely compromised immune system, making patients highly susceptible to opportunistic infections like MAC. The other thresholds, such as 200 cells/mm3 and 350 cells/mm3, are relevant for different opportunistic infections or management decisions but do not reflect the increased risk for MAC. Hence, identifying the correct threshold of under 50 cells/mm3 is crucial for clinicians to implement prophylactic measures and initiate treatment in those at high risk for MAC in the context of HIV.