The cyst with three visible nuclei and a central karyosome is most consistent with which organism?

• A. Endolimax nana
• B. Entamoeba coli
• C. Entamoeba histolytica
• D. Giardia duodenalis

Answer: (C)

A cyst's nuclear features are a primary way to tell amoebae apart. Entamoeba histolytica cysts typically have 1–4 nuclei and a small, central, compact karyosome with evenly distributed peripheral chromatin. Seeing three nuclei with a central karyosome fits this pattern, making Entamoeba histolytica the best match. Other species show different nuclear arrangements: Endolimax nana often has a large, less centrally located or irregular karyosome; Entamoeba coli usually has more nuclei (often many) and a coarser chromatin pattern; Giardia cysts have two to four nuclei but lack the central karyosome feature and have other characteristic structures.