A 25-year-old man presents with severe abdominal pain and shortness of breath. What is the most likely diagnosis if he has a perforated duodenal ulcer?

In this scenario, the presence of severe abdominal pain coupled with shortness of breath strongly suggests a perforated duodenal ulcer. When a duodenal ulcer perforates, it typically results in the leakage of gastric contents into the peritoneal cavity. This can lead to peritonitis, which causes significant abdominal pain as the body reacts to the inflammation and infection.

The shortness of breath noted in the patient can be attributed to referred pain or diaphragmatic irritation from the inflammatory process or fluid accumulation in the abdomen, which can affect respiratory function. Furthermore, the acute nature of the symptoms and their presentation align well with the diagnosis of a perforated ulcer, which is a surgical emergency.

Other options like tension pneumothorax, rectovesical fistula, or diaphragmatic hernia, while they can cause similar symptoms, would typically present with different clinical findings and perhaps a distinct history or mechanism of injury. In contrast, a perforated duodenal ulcer directly explains both the abdominal and respiratory symptoms in this patient.