**Core Concept**
The commonest site of peptic ulcers is determined by the balance between the acidic environment of the stomach and the protective mechanisms of the gastrointestinal mucosa. The duodenum, being the first part of the small intestine, is exposed to the acidic chyme from the stomach, making it a high-risk area for ulcer formation.

**Why the Correct Answer is Right**
The duodenum is the most common site of peptic ulcers due to the high concentration of gastric acid and pepsin in this region. The duodenal mucosa is relatively thin and lacks the protective mucous layer found in the stomach, making it more susceptible to acid damage. Additionally, the duodenum is a common site for Helicobacter pylori infection, which is a major cause of peptic ulcers.

**Why Each Wrong Option is Incorrect**
**Option A:** The stomach is not the most common site of peptic ulcers, although gastric ulcers can occur. The stomach has a thicker mucous layer and more effective protective mechanisms than the duodenum.
**Option B:** The jejunum is not a common site for peptic ulcers. The jejunum is farther down the small intestine and is less exposed to acidic chyme from the stomach.
**Option C:** The ileum is not a common site for peptic ulcers. The ileum is the last part of the small intestine and is not exposed to the same level of gastric acid as the duodenum.

**Clinical Pearl / High-Yield Fact**
The "Cushing's ulcer" is a type of duodenal ulcer that occurs in response to increased intracranial pressure, often due to head trauma or brain tumors. This type of ulcer is a classic exam trap and is worth remembering.

**Correct Answer:** D. The duodenum.