An iodine wet mount of a stool specimen shows an egg measuring about 110 μm with a large lateral projection. What is the most likely identification?

• A. Schistosoma haematobium
• B. Fasciolopsis buski
• C. Schistosoma mansoni
• D. Paragonimus westermani

Answer: (C)

A lateral spine on a helminth egg is a distinctive identifier. An egg around 110 μm with a large lateral projection fits Schistosoma mansoni well, because S. mansoni eggs are typically about this size and have a prominent lateral spine. This contrasts with Schistosoma haematobium eggs, which have a terminal (at the end) spine, and with Paragonimus westermani eggs, which are operculated and lack a lateral spine, and with Fasciolopsis buski eggs, which are large and thick-shelled without a distinctive lateral spine. Since the stool egg shows a large lateral spine and the size matches S. mansoni, the most likely identification is Schistosoma mansoni.