A 27 year old female graduate student recently returned from South America, where she completed a nature study of the rain forest. She spent months "living off the land." The woman went to her physician seeking treatment for a sinus infection, which she thought was responsible for several recent bouts of diarrhea. Upon questioning the patient, the doctor decided to collect stool for culture and parasitic examination. The stool culture was reported as "no enteric pathogens isolated." This suspicious form was seen on both wet preparations and on permanent stain. It measures 17 µm. The identify of this form is most likely:

Please select the single best answer