The description match with UC. Crohn would spare the rectum, tend to affect the terminal ileum, and have non-caseating granuloma in the crypt.

Amebic dysentery caused by E.histolytica would probably present similar with bloody diarrhea, and "flask-shaped" ulcers but the hx should include some risk factors such as traveler, MSM...

Ischemic bowel disease usually associates with atherosclerosis. Commonly see in elderly.