## **Core Concept**
The question tests knowledge on the diagnostic methods for Hydatid disease, specifically focusing on skin tests. Hydatid disease, also known as Echinococcosis, is a parasitic infection caused by *Echinococcus* species. The diagnosis often involves imaging and serological tests, but skin tests can also play a role.

## **Why the Correct Answer is Right**
The Casoni test, also known as the Casoni reaction, is a skin test used historically for the diagnosis of Hydatid disease. It involves injecting hydatid fluid into the skin and observing for an immediate hypersensitivity reaction. The test is considered positive if there is a wheal and flare reaction at the injection site. This reaction is indicative of an allergic response to the antigens present in the hydatid fluid, suggesting current or past exposure to *Echinococcus*.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not correspond to any widely recognized skin test for Hydatid disease.
- **Option B:** This option is incorrect as it does not accurately represent a known diagnostic skin test for Hydatid disease.
- **Option D:** This option is incorrect because, although the Montenegro test is a skin test, it is primarily used for the diagnosis of Leishmaniasis, not Hydatid disease.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while the Casoni test can be used for diagnosing Hydatid disease, it is not commonly used today due to the availability of more specific and sensitive serological tests and imaging techniques. However, it remains a historically significant test and may still be mentioned in exams.

## **Correct Answer:** . Casoni test