A psychotic, indigent man with a history of multi substance abuse has been involuntarily hospitalized for 1 week. Because of persistent diarrhea, stools are sent for ova and parasites, revealing numerous granular, spherical, thin-walled cysts measuring 10-20 mm in diameter. Trichrome stains show up to four nuclei in most of the cysts. These finding are consistent with an infection by which of the following organisms?
Cryptosporidium parvum Dientamoeba fragilis Entamoeba histolytica Giardia lamblia