Which organism has a trophozoite stage that is infectious to humans?

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

Answer: (B)

The key idea is that most intestinal protozoa transmit via a cyst that can survive outside the host, but some have infectious trophozoites instead. Dientamoeba fragilis is unique in that its trophozoite form is considered the infectious stage. There isn’t a reliably demonstrated cyst form for this organism, so infection begins when trophozoites are ingested and establish in the large intestine.

In contrast, the other organisms rely on cysts for transmission. Giardia duodenalis forms environmentally resistant cysts that are ingested to cause infection, while Endolimax nana and Entamoeba coli also transmit via cysts. Their trophozoites are the active, feeding forms inside the host but are not typically the infectious units themselves.

So, the organism with a trophozoite stage that is infectious to humans is Dientamoeba fragilis.