An iodine-stained amebic cyst shows three visible nuclei with a small central karyosome. Name the most likely organism.

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

Answer: (B)

Recognizing Entamoeba histolytica cysts hinges on nuclei count and the karyosome appearance. On iodine staining, mature E. histolytica cysts typically show up to four nuclei with a small, central karyosome and minimal peripheral chromatin. Seeing three visible nuclei with a small central karyosome fits this pattern, making Entamoeba histolytica the most likely organism.

Endolimax nana cysts, by contrast, often have a large, prominent irregular karyosome with little peripheral chromatin, not a small central one. Entamoeba coli cysts can have more nuclei (up to eight) and coarser peripheral chromatin, which differs from the small central karyosome pattern. Giardia duodenalis cysts are usually two-nucleated and have a distinct, different appearance.