**Core Concept**
Cushing ulcers are gastric mucosal ulcers caused by chronic use of high-dose corticosteroids. They are a rare but serious side effect of long-term steroid therapy, leading to mucosal thinning, decreased blood flow, and increased susceptibility to ulceration.

**Why the Correct Answer is Right**
The development of Cushing ulcers is related to the inhibitory effect of steroids on the production of gastric mucus and bicarbonate, leading to a decrease in the protective barrier of the gastric mucosa. This, combined with decreased blood flow and increased acid production, increases the risk of ulcer formation. The stomach, particularly the gastric antrum and fundus, is a common site for Cushing ulcers.

**Why Each Wrong Option is Incorrect**
**Option A:** The esophagus is not a common site for Cushing ulcers, as it is less affected by the direct effects of corticosteroids on the gastric mucosa.
**Option B:** The duodenum can be a site for Cushing ulcers, but it is not as commonly affected as the stomach.
**Option C:** The jejunum is not typically associated with Cushing ulcers, as it is more resistant to the effects of corticosteroids on the mucosa.

**Clinical Pearl / High-Yield Fact**
Cushing ulcers are a classic example of a medication-induced side effect, highlighting the importance of monitoring patients on long-term corticosteroid therapy for signs of gastrointestinal toxicity.

**Correct Answer: D. Jejunum.