A 52-year-old man from Delhi presents with a 6-month history of epigastric pain relieved by food and antacids. He has been taking ibuprofen daily for chronic back pain for the past 2 years. On examination, he appears pale and has mild epigastric tenderness. Laboratory investigations show hemoglobin 9.2 g/dL, MCV 72 fL, and fecal occult blood test positive. Upper endoscopy reveals a 1.5 cm ulcer on the anterior wall of the first part of the duodenum with a clean base. What is the most likely pathological mechanism responsible for ulcer formation in this patient?