**Core Concept:**
Duodenal ulcer is a type of gastric ulcer that occurs in the lining of the duodenum (second part of the small intestine). The most common cause of duodenal ulcers is the presence of Helicobacter pylori (H. pylori) infection, which can lead to inflammation and damage to the protective mucus layer in the stomach and duodenum, resulting in ulceration. Additionally, the use of nonsteroidal anti-inflammatory drugs (NSAIDs) is another common cause of duodenal ulcers by damaging the protective mucus layer and increasing acid production.

**Why the Correct Answer is Right:**
The correct answer, "H. pylori" (option C), is right because it is a bacterium that colonizes the stomach and duodenum, causing chronic inflammation and damage to the mucosal layer, which leads to ulcer formation. The correct answer also includes the use of NSAIDs (option D) as a common cause due to their ability to inhibit cyclooxygenase enzymes, which are responsible for producing prostaglandins that help protect the stomach lining from acid and pepsin damage.

**Why Each Wrong Option is Incorrect:**
Option A ("H. pylori infection") is incorrect because although H. pylori infection is a major cause of duodenal ulcers, it is not the only cause.
Option B ("NSAIDs") is incorrect because while NSAIDs are a common cause of duodenal ulcers, H. pylori infection is also a significant contributing factor.
Option E ("Alcohol") is incorrect because alcohol consumption may exacerbate existing ulcers but is not the primary cause.

**Clinical Pearl:**
In cases of suspected duodenal ulcers, it is essential to consider the role of H. pylori infection and NSAIDs usage in the differential diagnosis, along with other potential causes like alcohol and smoking. Early detection and management of these factors can prevent complications and improve patient outcomes. Moreover, appropriate antibiotic therapy and proton pump inhibitors can be used to treat H. pylori infection, while antacids and proton pump inhibitors can be used to manage NSAID-induced ulcers.