A trichrome stain from a fecal smear identifies a parasite about 15 μm in diameter. Which organism is this?

A trichrome stain from a fecal smear identifies a parasite about 15 μm in diameter. Which organism is this?

• A. Entamoeba hartmanni
• B. Dientamoeba fragilis
• C. Iodamoeba bütschlii
• D. Blastocystis hominis

Answer: (D)

A parasite seen at about 15 μm on a trichrome-stained fecal smear best fits Blastocystis hominis. Blastocystis commonly appears as relatively large, round bodies with a prominent central vacuole and peripheral cytoplasm, often around 10–20 μm in size. The other organisms listed are typically smaller in routine stool smears (Entamoeba hartmanni is a small Entamoeba, Dientamoeba fragilis trophozoites are also small, and Iodamoeba bütschlii forms are not characteristically this large on trichrome). So the observed size aligns most closely with Blastocystis hominis.

A parasite seen at about 15 μm on a trichrome-stained fecal smear best fits Blastocystis hominis. Blastocystis commonly appears as relatively large, round bodies with a prominent central vacuole and peripheral cytoplasm, often around 10–20 μm in size. The other organisms listed are typically smaller in routine stool smears (Entamoeba hartmanni is a small Entamoeba, Dientamoeba fragilis trophozoites are also small, and Iodamoeba bütschlii forms are not characteristically this large on trichrome). So the observed size aligns most closely with Blastocystis hominis.