Hydatid cysts in humans arise after ingestion of a tapeworm stage normally found in canines. Which stage initiates this cyst formation?

• A. Embryonated egg
• B. Cercaria
• C. Cysticercus
• D. Cercocystis

Answer: (A)

Hydatid disease is caused when humans accidentally ingest the eggs shed by dogs carrying a tapeworm. The stage that starts the cyst-forming process is the embryonated egg. Once swallowed, this egg releases the larval oncosphere, which penetrates the intestinal wall and travels to the liver or lungs, where it develops into the hydatid cyst that characterizes the infection. The other terms listed belong to different parasites or life cycles (for example, cercaria is a stage of flukes, cysticercus is a Taenia larval form, and cercocystis is not a standard stage in this cycle), so they don’t initiate hydatid cyst formation in humans.