**Core Concept**
The gastric outlet obstruction in peptic ulcer disease involves the pathophysiology of peptic ulcers and the anatomical structures of the gastrointestinal tract. Gastric outlet obstruction typically occurs when a peptic ulcer erodes through the muscularis mucosae of the duodenum or the pyloric channel, leading to narrowing or complete blockage of the gastric outlet.

**Why the Correct Answer is Right**
The correct answer is the duodenum, specifically the first part of the duodenum. This is because peptic ulcers are most commonly found in the duodenum, particularly in the duodenal bulb, and can cause gastric outlet obstruction by eroding through the muscularis mucosae and narrowing the lumen. The duodenum is the site of gastric outlet obstruction in approximately 90% of cases.

**Why Each Wrong Option is Incorrect**
**Option A:** The stomach is not a typical site for gastric outlet obstruction in peptic ulcer disease, as this condition primarily affects the duodenum and pyloric channel.

**Option B:** The jejunum is a part of the small intestine that is not typically involved in peptic ulcer disease or gastric outlet obstruction.

**Option C:** The liver is an organ that is not directly involved in the pathophysiology of peptic ulcer disease or gastric outlet obstruction.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that gastric outlet obstruction in peptic ulcer disease is often a late complication of chronic peptic ulcer disease. Patients may present with symptoms such as nausea, vomiting, abdominal pain, and weight loss.

**Correct Answer:** C.