**Core Concept:** Peritonitis is a life-threatening condition that develops when peritoneal cavity contents, such as bacteria, enter the abdominal cavity. It is a complication of perforated ulcers, particularly in the gastrointestinal tract.

**Why the Correct Answer is Right:** In this scenario, the patient presents with severe abdominal pain and has a perforating ulcer in the posterior wall of the stomach. The peritonitis would most likely develop initially due to the direct communication between the stomach and the peritoneal cavity, allowing bacteria from the stomach to enter the peritoneal cavity.

**Why Each Wrong Option is Incorrect:**

A. Peritonitis might develop in the small intestine: Although the patient has a stomach ulcer, the correct answer is based on the communication between stomach and peritoneal cavity, not specific to the small intestine.

B. Peritonitis developing in the colon is incorrect because the perforating ulcer is in the stomach, not the colon.

C. Peritonitis developing in the gallbladder is irrelevant in this context since the perforating ulcer is located in the stomach, not the gallbladder.

D. Peritonitis developing in the mesentery is not the correct answer, as the perforating ulcer is in the stomach, not the mesentery.

**Clinical Pearl:** To prevent peritonitis due to perforating gastrointestinal tract ulcers, it is essential to diagnose and treat the ulcers promptly, usually with antibiotics and antacids, as well as addressing the underlying cause (e.g., H. pylori infection or nonsteroidal anti-inflammatory drug use). Surgery may be required in severe cases.

**Correct Answer:** D. Peritonitis developing in the mesentery