**Core Concept**
H. pylori infection is a major cause of peptic ulcers, including duodenal ulcers. The bacterium adheres to the gastric mucosa and causes chronic inflammation, leading to ulcer formation. Infection rates and prevalence vary geographically.

**Why the Correct Answer is Right**
The correct answer is 60% because H. pylori infection is a significant risk factor for duodenal ulcers. Studies have shown that approximately 60% of duodenal ulcers are associated with H. pylori infection. The bacterium's presence triggers an immune response, leading to chronic inflammation and ulcer formation. The exact mechanism involves the production of cytotoxin-associated gene A (CagA) and vacuolating cytotoxin (VacA), which damage gastric epithelial cells.

**Why Each Wrong Option is Incorrect**
**Option A:** 5% is an underestimation of the actual risk. H. pylori infection is a common cause of duodenal ulcers worldwide.
**Option B:** 20% is too low, considering the bacterium's widespread prevalence in populations with duodenal ulcers.
**Option D:** 95% is an overestimation, as other factors, such as NSAID use and stress, can contribute to ulcer formation.

**Clinical Pearl / High-Yield Fact**
It is essential to test for H. pylori in patients with duodenal ulcers, as eradication therapy can significantly reduce the risk of recurrence. A combination of urea breath test, stool antigen test, or endoscopic biopsy with rapid urease test can confirm the diagnosis.

✓ Correct Answer: C. 60%