A 27 year old male was seen in the emergency room complaining of diarrhea. He had no other symptoms and was in very good health. The doctor ordered a stool for culture and parasite study. The culture revealed no enteric pathogens. This suspicious form was seen on the wet preparations and the permanent stain. It measured 16 µm. With which of the following parasites is this patient most likely infected?

Please select the single best answer