A 48-year-old male presents with a 5-year history of intermittent upper abdominal pain. Over the past 3 months, the epigastric pain has worsened, accompanied by nausea and decreased appetite. Barium meal X-ray examination reveals a 2.5cm*3.0cm niche shadow (ulcer crater) with irregular margins in the antrum of the stomach. Fecal occult blood test (FOBT) was positive on 3 occasions. The most likely diagnosis is: