Hydatid cyst is caused by
**Core Concept**
Hydatid cyst, also known as echinococcosis, is a parasitic infection caused by the larval stage of a tapeworm. The disease primarily affects the liver, but it can also involve other organs such as the lungs, brain, and kidneys. The parasite is typically transmitted through contact with contaminated feces or urine of an infected animal, usually dogs or sheep.
**Why the Correct Answer is Right**
The correct answer is caused by the Echinococcus granulosus parasite, a type of cestode. The parasite's life cycle involves a definitive host (dogs or sheep) and an intermediate host (humans or other animals). The adult tapeworm resides in the small intestine of the definitive host, where it produces eggs that are shed in the feces. Humans become infected when they accidentally ingest the eggs, which then develop into larval stages in the liver or other organs. The larva forms a cyst that can grow and cause complications if left untreated.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the correct answer is Echinococcus granulosus, not Echinococcus multilocularis, which causes alveolar echinococcosis.
**Option B:** This option is incorrect because the correct answer is not Taenia solium, which is a different type of tapeworm that causes cysticercosis.
**Option C:** This option is incorrect because the correct answer is not Trichinella spiralis, which is a type of nematode that causes trichinosis.
**Option D:** This option is incorrect because the correct answer is not Toxoplasma gondii, which is a type of protozoan that causes toxoplasmosis.
**Clinical Pearl / High-Yield Fact**
It's essential to note that hydatid cysts can be diagnosed using imaging studies, such as ultrasound or CT scans, and that treatment typically involves surgical removal of the cyst and administration of antiparasitic medications.
**Correct Answer:** C.