**Core Concept**
Hydatid disease, also known as echinococcosis, is a zoonotic parasitic infection caused by the tapeworm Echinococcus granulosus. The life cycle of this parasite involves a complex interplay between definitive and intermediate hosts. Understanding the role of the intermediate host is crucial in the transmission dynamics of this disease.

**Why the Correct Answer is Right**
Man serves as the intermediate host for Echinococcus granulosus. When an individual ingests contaminated food or water, the eggs of the parasite hatch in the gastrointestinal tract, releasing oncospheres that penetrate the intestinal wall and migrate to various organs, such as the liver, lungs, or brain. The oncospheres then form cysts, which can cause significant morbidity and mortality. The intermediate host does not complete the life cycle of the parasite but provides a suitable environment for the parasite to grow and reproduce.

**Why Each Wrong Option is Incorrect**
**Option B:** The dog is the primary definitive host for Echinococcus granulosus, where the adult worms reside in the small intestine and produce eggs that are shed in the feces.
**Option C:** Cats can be infected with Echinococcus multilocularis, a different species of tapeworm that causes alveolar echinococcosis, but they are not the intermediate host for E. granulosus.
**Option D:** Foxes can also act as definitive hosts for Echinococcus granulosus, but they are not the primary intermediate host.

**Clinical Pearl / High-Yield Fact**
Man is not the only intermediate host for Echinococcus granulosus, but it is the most common and significant one in terms of public health impact. Other intermediate hosts, such as sheep and cattle, are also important in the transmission cycle of the parasite.

**✓ Correct Answer: A. Man**