An iodine wet mount from a formalin-concentrated stool would most likely demonstrate which of the following?

• A. Immature cyst of Dientamoeba fragilis
• B. Oocyst of Cryptosporidium sp
• C. Trophozoite stage of Endolimax nana
• D. Mature cyst of Escherichia coli

Answer: (D)

The test hinges on recognizing which protozoan cyst is reliably seen in a formalin-preserved stool when viewed with an iodine mount. Formalin concentration preserves protozoan cysts, and iodine helps highlight their internal features. The mature cyst of Entamoeba coli is a classic finding in this setting: it’s relatively large and contains multiple nuclei, with a thick wall that makes it stand out under light microscopy. This makes it the best match for a cyst seen on an iodine wet mount of a formalin-concentrated stool. Immature cysts of Dientamoeba fragilis are not commonly encountered; Cryptosporidium oocysts require special stains (acid-fast or DFA); Endolimax nana is typically seen as trophozoites in fresh material and is not reliably detected in fixed concentrates; and a “mature cyst of Escherichia coli” isn't biologically accurate since E. coli is a bacterium and does not form protozoan cysts (the item may be intended to refer to Entamoeba coli).