## Core Concept
Casoni's test, also known as the Casoni test or thermoprecipitation test, is a diagnostic tool historically used to detect **echinococcosis** (hydatid disease) caused by *Echinococcus* species. The test involves injecting hydatid fluid into the patient and observing for an immediate hypersensitivity reaction. The sensitivity and specificity of such tests are crucial for diagnosing parasitic infections.

## Why the Correct Answer is Right
The Casoni test works on the principle of **type I hypersensitivity reaction**, where the patient already sensitized to *Echinococcus* antigens reacts to the intradermal injection of hydatid fluid. This reaction is characterized by the release of histamine and other mediators from mast cells, leading to symptoms like wheal and flare, itching, and sometimes systemic reactions. The sensitivity of the Casoni test, which refers to its ability to correctly identify those with the disease (true positive rate), has been reported to vary but is generally considered to be around **50%**.

## Why Each Wrong Option is Incorrect
- **Option A:** This option suggests a sensitivity that is too low for any diagnostic test to be considered useful. A sensitivity of less than 50% would imply that the test misses more than half of the cases, making it unreliable for clinical decision-making.
- **Option B:** While closer, this option still underestimates the generally reported sensitivity of the Casoni test. If the test's sensitivity were only slightly above 50%, it might still not capture enough cases to be clinically useful, especially considering the availability of more accurate diagnostic methods today.
- **Option D:** This option suggests a very high sensitivity, which might not accurately reflect the Casoni test's performance. A sensitivity of 90% or higher would imply that the test rarely misses cases, which contradicts the historical limitations and variability in results reported for the Casoni test.

## Clinical Pearl / High-Yield Fact
A key point to remember is that while the Casoni test can be used as a diagnostic aid, its sensitivity and specificity are not as high as modern serological tests and imaging techniques. Therefore, it's often used in conjunction with other diagnostic methods. A classic clinical pearl is that the Casoni test's sensitivity can be influenced by factors like the patient's immune response and the stage of the disease.

**Correct Answer: C. 50%**