## **Core Concept**
The question assesses knowledge of mycobacterial infections in immunocompromised patients, specifically those with HIV/AIDS. The CD4 count is a critical indicator of immune status in HIV-positive individuals. Certain mycobacteria are known to cause disease in individuals with compromised immune systems.

## **Why the Correct Answer is Right**
The correct answer, , is *Mycobacterium avium complex (MAC)*. MAC is known to cause disseminated infections in HIV-positive patients, particularly when the CD4 count falls below 50 cells/μL. However, the question specifies a CD4 count of 600/cu.mm, which might seem relatively high for typical MAC infections. Despite this, MAC can still cause disease at higher CD4 counts, but it's less common. The key point here is that MAC can cause disease across a range of CD4 counts but is more typical at lower counts.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Mycobacterium tuberculosis* typically causes disease in HIV-positive patients across various CD4 counts but is not specifically known for causing disease at a CD4 count as high as 600/cu.mm in the context of being more immunogenic and not solely opportunistic like MAC.
- **Option B:** This option seems to be missing; however, if considering common mycobacteria, *Mycobacterium leprae* causes leprosy and primarily affects the skin and peripheral nerves, not typically associated with opportunistic infections in HIV at any CD4 count.
- **Option D:** This option seems to be missing; however, *Mycobacterium kansasii* and other non-tuberculous mycobacteria (NTM) can cause disease in immunocompromised patients but are less commonly associated with the specific scenario described compared to MAC.

## **Clinical Pearl / High-Yield Fact**
A crucial point to remember is that the risk of opportunistic infections increases as the CD4 count decreases. However, some pathogens like *Mycobacterium avium complex* can cause disease even at relatively higher CD4 counts in HIV-positive individuals, making vigilance across a range of immune statuses necessary.

## **Correct Answer:** .