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?