Which parasite lacks a cyst stage?

• A. Giardia lamblia
• B. Dientamoeba fragilis
• C. Entamoeba coli
• D. Entamoeba histolytica

Answer: (B)

The key idea is how parasites spread through their life cycle stages, especially whether a cyst form is produced. A cyst is a hardy, environmentally resistant stage that lets parasites survive outside the host and enable fecal-oral transmission. The organisms Giardia lamblia, Entamoeba histolytica, and Entamoeba coli all form cysts as part of their life cycles, so they are commonly transmitted via cyst-contaminated water or food. Dientamoeba fragilis, on the other hand, is traditionally described as lacking a true cyst stage and existing mainly as trophozoites in the intestinal lumen; this means it does not rely on environmental cysts for transmission, making its life cycle and diagnostic considerations distinct. Thus, the parasite that lacks a cyst stage is Dientamoeba fragilis.