**Core Concept**
The question tests the understanding of mycobacterial infections in immunocompromised patients, specifically those with HIV. **Mycobacterium tuberculosis** and non-tuberculous mycobacteria (NTM) can cause disease in HIV-positive individuals, but the risk and type of mycobacterial infection vary with the CD4 count.

**Why the Correct Answer is Right**
Given the high CD4 count of 600/cu.mm, the patient is less immunocompromised, and thus, more likely to be infected by **Mycobacterium tuberculosis**, which can affect individuals with relatively preserved immune function.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is not specified, and without the name, we cannot determine its accuracy.
**Option B:** Incorrect for similar reasons as Option A.
**Option C:** Incorrect as it is also unspecified.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **Mycobacterium avium complex (MAC)** typically causes disease in HIV-positive patients with a CD4 count below 50/cu.mm, making it less likely in this scenario with a CD4 count of 600/cu.mm.

**Correct Answer:** D. Mycobacterium tuberculosis.