Which organism has canids as definitive hosts and is a source of hydatid cysts in humans?

• A. Taenia saginata
• B. Taenia solium
• C. Dipylidium caninum
• D. Echinococcus granulosus

Answer: (D)

The key idea is the life cycle where canids act as the definitive hosts and humans can become accidental intermediate hosts, leading to hydatid disease. Echinococcus granulosus fits this pattern: the adult tapeworm lives in canids (dogs and related species) and sheds eggs in their feces. People ingest these eggs through contaminated food, water, or hands, and the larvae migrate to organs like the liver and lungs, where they form hydatid cysts. These cysts grow slowly and can rupture, causing serious complications, so recognizing this parasite explains both the source of infection (canids) and the human disease (hydatid cysts).

Taenia saginata and Taenia solium are beef and pork tapeworms, respectively. Humans can be definitive hosts for these, but they do not produce hydatid cysts in humans. Dipylidium caninum can involve canids as definitive hosts and is acquired by humans occasionally, but it does not form hydatid cysts in human tissues. So the organism that aligns with both canids as definitive hosts and the source of hydatid cysts in humans is Echinococcus granulosus.