A 17 year old female went to her doctor complaining of diarrhea. With the exception of seasonal allergies, she was in relatively good health. Patient history was unremarkable. A stool was submitted for culture and parasite examination. The culture was reported out as "no enteric pathogens isolated." This suspicious form was seen on wet preparation and permanent stain. It measures 27 µm. This patient is most likely suffering from which of the following conditions:

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