3 Possible diagnoses for the above cyst:
These 3 organisms are morphologically identical, yet genetically distinct, and only E. histolytica has definitely been shown to cause human disease. I also would have accepted immature E. coli. E. hartmanni would not be included in the differential given the size of these organisms (15um).
I'll say a few words on E. moshkovskii since most of you probably haven't heard of it before. Even though it was first described 1941, it was thought at that time to be only an environmental organism. However, it has now been identified in human fecal samples worldwide. The true prevalence is not well understood, and this is partially because it cannot be distinguished from E. histolytica and E. dispar by light microscopy. There is some evidence emerging that it may be associated with human disease, so I'm sure we will continue to hear more about it in the future.