A trophozoite about 12 μm in size with two nuclei and no cyst stage corresponds to which organism?

• A. Entamoeba hartmanni
• B. Entamoeba coli
• C. Entamoeba histolytica
• D. Dientamoeba fragilis

Answer: (D)

A trophozoite around 12 μm with two nuclei and no cyst stage points to Dientamoeba fragilis. The key identifying feature of this organism is that its trophozoites are binucleate, typically about 5–12 μm in size, and no cyst form is observed in humans. Other Entamoeba species usually have either a single nucleus in the trophozoite or cysts with multiple nuclei (for example, cysts with four nuclei in Entamoeba histolytica, or cysts with up to eight nuclei in Entamoeba coli), and their trophozoites don’t characteristically present as binucleate without cysts. Thus, the combination of a ~12 μm binucleate trophozoite and the absence of a detectable cyst stage best fits Dientamoeba fragilis.