**Core Concept**
The question tests the ability to diagnose opportunistic infections in immunocompromised patients, specifically those with AIDS. **Cryptosporidiosis** is an infection caused by the protozoan parasite *Cryptosporidium*, which can cause severe diarrhea in individuals with weakened immune systems.

**Why the Correct Answer is Right**
*Cryptosporidium* is an acid-fast organism that can be identified by its characteristic cysts in stool samples. In patients with AIDS, the immune system is severely compromised, making them more susceptible to opportunistic infections like cryptosporidiosis. The acid-fast positive stain cysts in the stool examination are indicative of *Cryptosporidium* infection.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because *Isospora belli* is another opportunistic parasite, but it is typically identified by its larger oocysts.
**Option B:** Incorrect as *Cyclospora cayetanensis* is also an opportunistic parasite, but its oocysts are larger and have distinct features.
**Option D:** Incorrect because *Microsporidia* are obligate intracellular parasites that require special staining techniques for identification.

**Clinical Pearl / High-Yield Fact**
In immunocompromised patients, particularly those with AIDS, opportunistic infections like cryptosporidiosis can cause severe and prolonged diarrhea, leading to significant morbidity and mortality.

**Correct Answer:** D. Cryptosporidium.