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

A. Salmonella
B. Shigella
C. Yersinia
D. Campylobacter

**Core Concept:** Ye Casoni's test is an indirect fecal antigen test used to detect the presence of certain bacteria causing enteric infections. In this case, it indicates an infection with a Gram-negative bacillus.

**Why the Correct Answer is Right:** The correct answer is **C. Yersinia** because Yersinia enterocolitica is the most common cause of enterocolitis and reactive mesenteric lymphadenitis (Peyronie disease), leading to an abdominal mass and a positive Casoni's test. The other options are not typically associated with positive Casoni's test and abdominal mass.

**Why Each Wrong Option is Incorrect:**

A. Salmonella: While Salmonella can cause enterocolitis, it is less likely to cause reactive mesenteric lymphadenitis and a positive Casoni's test.
B. Shigella: Shigella is less likely to cause reactive mesenteric lymphadenitis and a positive Casoni's test compared to Yersinia.
D. Campylobacter: Campylobacter is less likely to cause reactive mesenteric lymphadenitis and a positive Casoni's test than Yersinia.

**Clinical Pearl:** Ye Casoni's test is a valuable diagnostic tool in identifying the etiological agent in cases of enterocolitis and reactive mesenteric lymphadenitis, especially for Yersinia enterocolitica infection.

**Correct Answer:** C. Yersinia