A 48-year-old male presents with intermittent abdominal pain and abdominal distension for over 20 years, worsening over the past 3 months. Bowel movements occur 4-5 times per day, slightly loose, with poor appetite and 5kg weight loss in 1 month. Laboratory tests show fecal occult blood (±) to (+). Hb: 104g/L. The best diagnostic method is: