Which protozoan parasite can cause severe diarrhea and dehydration?

Cryptosporidium parvum is the correct choice because it is a well-known protozoan parasite that causes cryptosporidiosis, which is characterized by severe diarrhea and can lead to significant dehydration. This parasite is particularly resilient and can survive in harsh environmental conditions, making it a common contaminant in water sources.

In immunocompetent individuals, the infection may cause relatively mild gastrointestinal distress; however, in immunocompromised individuals—such as those with HIV/AIDS—the disease can be chronic and life-threatening. The watery diarrhea caused by Cryptosporidium is one of its hallmark symptoms and can result in dehydration due to the loss of fluids and electrolytes.

In comparison, while Giardia lamblia also causes diarrhea, it tends to be less severe and is more often associated with a foul-smelling stool. Cyclospora cayetanensis and Entamoeba histolytica can indeed cause diarrhea and other gastrointestinal symptoms as well, but Cryptosporidium is especially notable for its link to severe, watery diarrhea, which leads to dehydration. Thus, in contexts such as outbreaks and in immunocompromised patients, Cryptosporidium parvum's impact is significant enough to make it a primary concern in parasitology.