**Question:** Which is true regarding hydatid cyst of lung?

A. Hydatid cysts are caused by the ingestion of eggs from contaminated food or water sources.
B. Hydatid cysts primarily affect the liver and lungs.
C. Echinococcus granulosus is the causative organism in hydatid cyst disease.
D. The clinical presentation of hydatid cyst is always acute and severe.

**Correct Answer: C. Echinococcus granulosus is the causative organism in hydatid cyst disease.**

**Core Concept:** Hydatid cysts are parasitic infections caused by the larval stage of Echinococcus granulosus, a tapeworm. The life cycle involves the intermediate host (e.g., sheep, goats) and the definitive host (e.g., dogs, wolves). Humans can become infected when they consume food or water contaminated with the tapeworm eggs.

**Why the Correct Answer is Right:** Echinococcus granulosus is the correct answer because it is the causative organism responsible for the development of hydatid cysts. The other options are incorrect for the following reasons:

A. Hydatid cysts can affect various organs, including the liver, lungs, and spleen, not just the liver and lungs (option B). The correct answer should mention Echinococcus granulosus as the causative organism (option C). Option D is incorrect because the clinical presentation of hydatid cyst disease can range from asymptomatic to symptomatic, with a broad spectrum of clinical manifestations.

**Why Each Wrong Option is Incorrect:**

B. Hydatid cysts affecting the liver and lungs is a general statement about the disease's sites of involvement, but it does not address the specific organism causing the disease. This makes option B incorrect. The correct answer (option C) identifies Echinococcus granulosus as the causative organism.

D. Hydatid cysts can present acutely, chronically, or subacutely, and the clinical presentation is not always severe. This makes option D incorrect. The correct answer (option C) identifies Echinococcus granulosus as the causative organism.

Echinococcus granulosus, a tapeworm, is responsible for the development of hydatid cysts in the human body. These cysts can affect various organs, not just the liver and lungs. The clinical presentation can range from asymptomatic to symptomatic, and the severity depends on factors such as cyst size, location, and complications.