**Question:** Malignant hydatid is caused by

A. Echinococcus multilocularis
B. Echinococcus granulosus
C. Taenia solium
D. Taenia saginata

**Correct Answer:** A. Echinococcus multilocularis

**Core Concept:** Malignant hydatid disease is a rare but severe form of echinococcosis, caused by the larval stages of tapeworms belonging to the genus Echinococcus.

**Why the Correct Answer is Right:** Malignant hydatid disease is caused by the larval stages of Echinococcus multilocularis, a parasite commonly found in the intestines of intermediate hosts such as foxes, cats, and other carnivorous mammals. The parasite's life cycle involves a definitive host (e.g., dogs, wolves), intermediate hosts, and a final host (humans). When humans ingest eggs from contaminated food (e.g., fruits, vegetables), the larva, named Alveolar echinococcosis, can develop.

**Why Each Wrong Option is Incorrect:**

A. Echinococcus multilocularis is responsible for Malignant hydatid, not Echinococcus granulosus (option B).
B. Echinococcus granulosus causes Cystic echinococcosis, a different form of echinococcosis affecting the liver, lungs, and other organs but not Malignant hydatid (option A).
C. Taenia solium causes Cysticercosis, a parasitic infection caused by pork tapeworms, which is different from Malignant hydatid (option C).
D. Taenia saginata causes Bovine cysticercosis, another parasitic infection caused by beef tapeworms, unrelated to Malignant hydatid (option D).

**Clinical Pearl:** Echinococcosis is a zoonotic infection caused by tapeworms of the genus Echinococcus, with two main forms: Alveolar echinococcosis (caused by Echinococcus multilocularis) and Cystic echinococcosis (caused by Echinococcus granulosus). The correct answer, Echinococcus multilocularis, leads to Malignant hydatid disease, a rare but severe form of echinococcosis that involves the lungs, liver, or other organs, whereas Echinococcus granulosus causes Cystic echinococcosis, affecting the liver, lungs, and other organs but not Malignant hydatid.