A 72-year-old man presents to the emergency department with a 2-day history of vomiting bright red blood. He reports feeling dizzy and weak. His past medical history includes hypertension, diabetes, and osteoarthritis. He has no history of liver disease or alcohol use. On physical examination, his blood pressure is 95/60 mmHg, heart rate is 110 beats per minute, and he appears pale. Abdominal examination reveals mild epigastric tenderness without rebound or guarding. Laboratory tests show a hemoglobin level of 7.5 g/dL (normal: 13.8-17.2 g/dL) and a blood urea nitrogen (BUN) level of 40 mg/dL (normal: 7-20 mg/dL). Question: What is the most likely diagnosis?