Hydatid (echinococcal) cyst of the liver is caused by which parasite?

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Hydatid cysts in the liver are caused by the tapeworm Echinococcus granulosus. In its life cycle, dogs are the definitive hosts and shed eggs in their feces, while sheep or other livestock serve as intermediate hosts. Humans become accidental hosts by ingesting eggs, which release oncospheres in the gut and travel through the portal circulation to the liver, the first major filter, where a cyst develops. Over time the cyst grows and may contain daughter cysts and scolices; if it ruptures, it can provoke an allergic reaction or spread the larvae.

This parasite specifically explains the hydatid (echinococcal) cyst of the liver. The other organisms listed cause different diseases—Giardia causes giardiasis; Entamoeba histolytica causes amebic dysentery (and can cause amebic liver abscess, not a classic hydatid cyst); Ascaris lumbricoides is an intestinal roundworm with different clinical features.