A 34-year-old male worker, previously healthy, has experienced acid regurgitation and epigastric discomfort when hungry for the past 2 months, along with melena in recent days. Physical examination shows mild anemic appearance, with no abnormalities in heart or lungs. Hemoglobin (Hb) is 90g/L, white blood cell (WBC) count is 9.0*10^9/L with normal differential. The most likely cause of his upper gastrointestinal bleeding (GI bleeding) is: