**Core Concept**
The Casoni test is a diagnostic tool used to detect infection caused by a specific parasitic organism. It is based on the principle of an immediate hypersensitivity reaction, where the patient's serum is mixed with an antigen derived from the parasite, leading to a localized skin reaction.

**Why the Correct Answer is Right**
The Casoni test is specifically used to detect infection caused by *Echinococcus granulosus*, the parasite responsible for cystic echinococcosis (also known as hydatid disease). When the patient's serum is mixed with the antigen, it triggers an immediate hypersensitivity reaction, resulting in a localized skin reaction at the injection site. This reaction is a sign of a previous exposure to the parasite and is used as a diagnostic marker for the disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** The Casoni test is not used to detect infection caused by *Toxocara canis*, a different parasitic organism that causes visceral larva migrans.
* **Option B:** The Casoni test is not used to detect infection caused by *Taenia saginata*, a parasitic organism that causes beef tapeworm infection.
* **Option D:** The Casoni test is not used to detect infection caused by *Trichinella spiralis*, a parasitic organism that causes trichinosis.

**Clinical Pearl / High-Yield Fact**
The Casoni test is a classic example of an immediate hypersensitivity reaction, which can be used as a diagnostic marker for certain parasitic infections. It is essential to remember that this test is specific to *Echinococcus granulosus* and should not be used to diagnose other parasitic infections.

**Correct Answer: C. Echinococcus granulosus**