A 52-year-old man from Delhi presents with a 3-month history of epigastric pain relieved by food and antacids. He reports black tarry stools for the past 2 weeks. On examination, he is pale with a blood pressure of 100/65 mmHg and heart rate of 98/min. Hemoglobin is 9.2 g/dL. Upper endoscopy reveals a 1.5 cm ulcer in the first part of the duodenum with a visible vessel at the base. The ulcer has a clean base with minimal surrounding edema. H. pylori serology is negative, and he denies NSAID use. What is the most likely pathological mechanism underlying this ulcer?