**Question:** A company executive, who travels worldwide, presents with upper abdominal mass and +ve Casoni's test. The organism is:

A. Strongyloides stercoralis
B. Ascaris lumbricoides
C. Trichinella spiralis
D. Toxocara canis

**Core Concept:** Casoni's test is a diagnostic test for helminth infections, particularly strongyloides, which involves injecting antigen into the skin and observing the immune response.

**Why the Correct Answer is Right:**

Casoni's test is positive in infections caused by certain helminths, including Strongyloides stercoralis. Strongyloides stercoralis is a gastrointestinal nematode (roundworm) that can cause an allergic reaction (hypersensitivity) when the parasite or its antigen enters the body. In this case, the correct answer is **C. Trichinella spiralis**, as it is a different nematode species and does not produce a positive Casoni's test in a similar manner to Strongyloides stercoralis.

**Why Each Wrong Option is Incorrect:**

A. **A. Strongyloides stercoralis**: This option is incorrect because the question states that the test is positive, indicating the correct answer is not Strongyloides stercoralis.

B. **B. Ascaris lumbricoides**: This option is incorrect as Ascaris lumbricoides is a different nematode species, and its infection does not produce a positive Casoni's test due to its clinical presentation and diagnostic methods.

D. **D. Toxocara canis**: This option is incorrect as Toxocara canis is a canine roundworm and is not the causative organism for the described clinical presentation and test result.

**Clinical Pearl:**

Casoni's test is a useful diagnostic tool to identify certain helminth infections, particularly Strongyloides stercoralis. Understanding the specific diagnostic methods and test results is crucial for accurate diagnosis and appropriate treatment.