Which parasite is best detected by observing motility and gross morphology on direct wet mount?

• A. Entamoeba histolytica
• B. Plasmodium vivax
• C. Giardia lamblia
• D. Taenia saginata

Answer: (C)

Motility on a fresh stool wet mount is the best clue for Giardia lamblia because its trophozoite is highly active and has a distinctive pear-shaped body with multiple flagella, plus a ventral adhesive disk. When examined in saline soon after collection, you can see the rapid, characteristic “falling leaf” movement and the general morphology that makes Giardia stand out from other parasites. Cysts are non-motile and require concentration and permanent stains to identify, so they aren’t as helpful on a direct wet mount. Other organisms are detected by different methods or stages: Plasmodium vivax is seen in stained blood smears, Taenia saginata eggs or proglottids require stool concentration and different stains, and Entamoeba histolytica trophozoites can be seen in fresh stool but are less reliably distinguished by motility alone. Thus, Giardia lamblia is the organism best detected by observing motility and gross morphology on direct wet mount.