**Question:** Skin test useful in hydatid disease is

A. Patch test
B. Prick test
C. Serum specific immunoglobulin E (IgE)
D. Indirect haemagglutination test

**Correct Answer:** C. Serum specific immunoglobulin E (IgE)

**Core Concept:** Hydatid disease is a zoonotic infection caused by the larval form of the tapeworm Echinococcus granulosus. Humans can develop cysts in various organs, including the liver, lungs, and kidneys.

**Why the Correct Answer is Right:** The skin test used for hydatid disease is determined by the presence of specific antibodies, particularly immunoglobulin E (IgE), in the patient's blood serum. The IgE antibodies are produced in response to the parasite's antigens and can be detected using a blood test. This serological test is crucial for diagnosing hydatid disease as it identifies the host's immune response to the parasite.

**Why Each Wrong Option is Incorrect:**

A. Patch test: This test is used to diagnose allergies, not infections. It involves applying allergen patches on the skin for 48-72 hours and monitoring for a reaction, which is not applicable to hydatid disease diagnosis.

B. Prick test: Similar to the patch test, the prick test is used to diagnose allergies and not infections like hydatid disease. It involves pricking the skin with a drop of allergen solution, which is not relevant for diagnosing hydatid disease.

D. Indirect haemagglutination test: This test, also known as the enzyme-linked immunosorbent assay (ELISA), is a serological test used to measure the presence of specific antibodies, including IgE, in the blood serum. While it is a valid diagnostic tool, it is not specific to hydatid disease and can be positive in other infections as well.

**Clinical Pearl:** The most appropriate skin test for hydatid disease is the Indirect Haemagglutination test (IHAT) or Enzyme-Linked Immunosorbent Assay (ELISA) to detect specific antibodies, including IgE. These tests provide a comprehensive understanding of the host's immune response to the parasite, making them useful in confirming the diagnosis of hydatid disease.