**Core Concept**
The pathogenesis of duodenal ulcers involves an imbalance between the protective mechanisms of the gastric mucosa and the aggressive factors that impair its integrity. This imbalance is primarily caused by the overproduction of acid and pepsin due to the presence of Helicobacter pylori or the use of nonsteroidal anti-inflammatory drugs (NSAIDs).

**Why the Correct Answer is Right**
The most common cause of duodenal ulcers is the bacterium Helicobacter pylori, which colonizes the gastric mucosa and impairs the protective mechanisms of the mucosa through various virulence factors. H. pylori infection leads to chronic gastritis, which results in the overproduction of acid and pepsin, causing mucosal damage and ulcers. The bacterium also induces the production of cytokines, such as interleukin-1 beta (IL-1β), which further exacerbates the inflammatory response.

**Why Each Wrong Option is Incorrect**
* **Option A:** Smoking is a risk factor for duodenal ulcers, but it is not the most common cause. Smoking can impair the protective mechanisms of the gastric mucosa and increase the risk of ulcers, but it is not as significant as H. pylori infection.
* **Option B:** Stress is often cited as a cause of duodenal ulcers, but it is not a direct cause. Stress can exacerbate the symptoms of ulcers, but it is not a primary factor in their development.
* **Option C:** The use of NSAIDs is a common cause of gastric ulcers, but it is not the most common cause of duodenal ulcers. NSAIDs can impair the protective mechanisms of the gastric mucosa and cause ulcers, but H. pylori infection is more prevalent.

**Clinical Pearl / High-Yield Fact**
The eradication of H. pylori through antibiotics and acid suppression therapy is the primary treatment for duodenal ulcers. The use of NSAIDs should be avoided, and patients should be advised to quit smoking to reduce their risk of developing ulcers.

**Correct Answer: C. Stress is often cited as a cause of duodenal ulcers, but it is not a direct cause. Stress can exacerbate the symptoms of ulcers, but it is not a primary factor in their development.