Which organism would be routinely considered nonpathogenic?

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

Answer: (B)

Not all intestinal protozoa cause disease; some are considered harmless residents of the gut. Endolimax nana is classically viewed as nonpathogenic because it colonizes the large intestine without invading tissue or provoking inflammation, so it’s typically an incidental finding in stool that doesn’t require treatment.

In contrast, Entamoeba histolytica has invasive forms that penetrate the intestinal lining and can cause dysentery and liver abscesses. Giardia duodenalis attaches to the small-intestinal mucosa and disrupts absorption, leading to diarrhea and malabsorption. Dientamoeba fragilis is associated with symptoms in many cases and is generally considered pathogenic in modern practice, though its disease-causing potential is less straightforward than the others.