**Core Concept**
Cryptosporidium is a genus of protozoan parasites that cause cryptosporidiosis, a diarrheal disease in humans. The parasite is transmitted through the fecal-oral route, contaminating water and food sources.

**Why the Correct Answer is Right**
The correct answer is related to the characteristics of Cryptosporidium. Cryptosporidium species, such as Cryptosporidium parvum, are known to cause severe diarrhea in immunocompromised individuals, including those with HIV/AIDS. The parasite invades the microvilli of the small intestine, leading to malabsorption of nutrients and electrolytes.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because Cryptosporidium is indeed a protozoan parasite, making this statement true.
**Option B:** Incorrect because Cryptosporidium parvum is a well-documented cause of cryptosporidiosis, making this statement true.
**Option C:** Incorrect because Cryptosporidium can cause severe diarrhea in immunocompromised individuals, making this statement true.

**Clinical Pearl / High-Yield Fact**
Remember that Cryptosporidium parvum is highly resistant to chlorine disinfection, making it a significant concern for waterborne outbreaks.

**Correct Answer: A.**