Answer: Amoebiasis, due to Entamoeba histolytica
The diagnosis can be made by identification of classic Entamoeba trophozoites with a "ring and dot" nucleus (see image) containing engulfed red blood cells (top 2 arrows).
This protozoan parasite only rarely invades the intestinal wall and causes symptomatic infection. Infection is most commonly asymptomatic. This organism is morphologically identical to the closely related human commensals E. dispar and E. moshkovskii, which may also be found in the human intestinal tract. Unless invasive disease or trophozoites containing ingested red blood cells are observed in biopsy or stool preparations, these amebae cannot be differentiated morphologically.