The ingestion of red blood cells within a trophozoite is most diagnostic of which parasite?

• A. Entamoeba coli
• B. Endolimax nana
• C. Iodamoeba butschlii
• D. Entamoeba histolytica

Answer: (D)

The key idea is that trophozoites that contain ingesting red blood cells are a hallmark of a specific invasive parasite. Entamoeba histolytica is the pathogenic species that invades the intestinal lining and phagocytoses host cells, including red blood cells, during tissue invasion. Seeing red blood cells inside a trophozoite in a stool sample strongly supports infection with Entamoeba histolytica rather than a nonpathogenic intestinal amoeba. The other choices—Entamoeba coli, Endolimax nana, and Iodamoeba butschlii—are typically nonpathogenic and do not show erythrophagocytosis within trophozoites, so they don’t provide this diagnostic cue.