Which organism would routinely be considered a nonpathogen?

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

Answer: (A)

The key idea is distinguishing organisms that live in the gut without causing disease from those that do. Endolimax nana is a small intestinal amoeba that typically exists as a harmless commensal in human stool and does not invade tissue or trigger illness. Because it is not associated with disease in routine cases, it is routinely considered a nonpathogen. In contrast, Entamoeba histolytica can invade the intestinal wall and cause amebic colitis and liver abscesses, Giardia duodenalis leads to giardiasis with malabsorption and diarrhea, and Dientamoeba fragilis is often linked to symptoms in some patients, so they are regarded as pathogenic or potentially pathogenic. Therefore Endolimax nana is the nonpathogen.