Answer: Amebiasis. This disease is caused by j
E. histolytica, a protozoan parasite.
Most of you recognized the classic "flask shaped" ulcer at low magnification (below).
This type of ulcer is so named because its base is wider than its apex.
At higher magnification, numerous trophozoites can be seen at the leading edge of the ulcer. Under oil, the nuclear features are difficult to make out, but you can tell this is the "ring and dot" chromatin pattern or E. histolytica and that there is an ingested RBC in the cytoplasm.
Heather mentions that histologic sections are particularly difficult to interpret for parasitologists who are not used to seeing them. That's definitely true. While I don't have any methods for interpreting all different types of tissue sections, I will say that it gets easier with time, as you become familiar with some of the key features for each parasite. In this case, the "flask-shaped ulcer" and trophozoites with ingested RBCs are diagnostic. Don't give up! I'll keep pointing out the key features, and you will get used to seeing a spectrum of histopathology over time.