Which statement best characterizes Entamoeba histolytica trophozoites in stool?

• A. They are always noninvasive
• B. They may contain ingested red blood cells
• C. They are resistant to acid-fast staining
• D. They have multiple flagella

Answer: (B)

The main idea is that pathogenic Entamoeba histolytica trophozoites in stool can be identified by their ability to ingest red blood cells. This phagocytosis happens as the parasite invades the intestinal mucosa and phagocytoses host cells, so seeing red blood cells inside a trophozoite strongly points to E. histolytica rather than nonpathogenic amoebae. This RBC ingestion reflects its invasive behavior and is a key microscopic clue used to differentiate it from species like Entamoeba dispar, which do not ingest red blood cells in trophozoites.

Trophozoites of Entamoeba histolytica move by pseudopodia and do not have flagella, so a statement about multiple flagella is incorrect. They are not characteristically resistant to acid-fast staining; these organisms are not typically identified by acid-fast methods, and the presence of ingested red blood cells is the more diagnostic feature when stool trophozoites are seen. They are capable of invasion, so describing them as always noninvasive would be misleading.