**Core Concept**
Cryptosporidiosis is a diarrheal disease caused by the protozoan parasite *Cryptosporidium*, which infects the small intestine. This parasite is known to cause severe diarrhea in immunocompromised individuals, particularly those with HIV/AIDS.

**Why the Correct Answer is Right**
The diagnosis is based on the presence of oocysts in the stool, which are the infectious form of the parasite. The oocysts are shed in the stool and can be detected by microscopic examination. In immunocompromised individuals, such as HIV-positive individuals, cryptosporidiosis can cause severe and persistent diarrhea, leading to dehydration and weight loss. The parasite invades the microvilli of the small intestine, causing damage to the intestinal epithelium and leading to malabsorption.

**Why Each Wrong Option is Incorrect**
* **Option A:** Giardiasis is caused by *Giardia lamblia*, which is a different type of protozoan parasite. While giardiasis can also cause diarrhea, the oocysts of *Giardia* are larger (10-20 microns) and have a different morphology.
* **Option B:** Cystoisosporiasis is caused by *Cystoisospora belli*, which is another type of protozoan parasite. However, the oocysts of *Cystoisospora* are larger (20-30 microns) and have a different morphology.
* **Option D:** Microsporidiosis is caused by *Microsporidia*, which are a group of obligate intracellular parasites. However, the oocysts of *Microsporidia* are much smaller (1-4 microns) and are detected using specialized staining techniques.

**Clinical Pearl / High-Yield Fact**
In immunocompromised individuals, such as HIV-positive individuals, diarrhea is a common symptom of opportunistic infections. Cryptosporidiosis is a significant cause of diarrhea in this population, and early diagnosis is essential to initiate treatment.

**Correct Answer:** C.