Duodenal ulcers are open sores that develop on the lining of the duodenum, the first part of the small intestine, often caused by Helicobacter pylori infection or the prolonged use of nonsteroidal anti-inflammatory drugs (NSAIDs). They can lead to symptoms such as abdominal pain, bloating, and nausea, and are typically diagnosed through endoscopy and treated with a combination of antibiotics, acid-suppressing medications, and lifestyle changes.