**Core Concept**
A posteriorly perforating ulcer in the pyloric antrum of the stomach is a type of perforated peptic ulcer, which can lead to the leakage of gastric contents into the peritoneal cavity. The initial response to this leakage is the formation of a localized peritonitis or abscess, due to the accumulation of infectious material in a contained space.

**Why the Correct Answer is Right**
The correct answer is based on the anatomy of the peritoneal cavity and the location of the perforation. The pyloric antrum is located near the pyloric sphincter, which is the transition zone between the stomach and the duodenum. A posterior perforation in this area is likely to occur near the gastroduodenal artery, which runs along the superior border of the duodenum. The initial leakage of gastric contents is likely to be contained by the omentum, a fold of peritoneal tissue that hangs down from the greater curvature of the stomach. The omentum can trap the infectious material, leading to the formation of a localized abscess or peritonitis.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the omentum is more likely to trap the infectious material and contain the leakage, rather than allowing it to spread freely into the general peritoneal cavity.
* **Option B:** This option is incorrect because the pancreas is not directly adjacent to the pyloric antrum, and the leakage of gastric contents is less likely to be contained by the pancreatic tissue.
* **Option C:** This option is incorrect because the liver is not directly adjacent to the pyloric antrum, and the leakage of gastric contents is less likely to be contained by the liver tissue.
* **Option D:** This option is incorrect because the general peritoneal cavity is not the initial site of abscess formation, but rather the site of widespread peritonitis.

**Clinical Pearl / High-Yield Fact**
In cases of perforated peptic ulcer, the omentum plays a crucial role in containing the leakage and preventing the spread of infection. The omentum can trap the infectious material and form a localized abscess, which can be managed surgically or conservatively.

**Correct Answer:** Omental bursa (lesser sac).