Hydatid cysts are caused by which organism?

Hydatid cysts are specifically caused by the Echinococcus species, most commonly Echinococcus granulosus. This tapeworm's larvae can form cysts in various organs, particularly the liver and lungs of intermediate hosts, including livestock and humans. The lifecycle of Echinococcus involves dogs as definitive hosts, which shed the eggs in their feces. When these eggs are ingested by humans or other intermediate hosts, they hatch in the intestine and the larvae penetrate the intestinal wall, subsequently forming hydatid cysts in internal organs.

In contrast, the other organisms listed do not cause hydatid cysts. Giardia is a protozoan responsible for giardiasis, a gastrointestinal infection. Cryptosporidium also causes gastrointestinal illness, particularly in immunocompromised individuals, but it does not lead to cyst formation typical of Echinococcus. Ascaris is a helminth that causes ascariasis, primarily affecting the intestinal tract, without forming cysts in organs as seen with Echinococcus. Therefore, Echinococcus is specifically linked to the formation of hydatid cysts, making it the correct answer.