A 52-year-old man has suffered with chronic diarrhea for several years, but has refused to see a doctor. He finally comes because he is having trouble driving at night, because of difficulty seeing. Physical examination reveals a slender, pale, unwell-looking man. He has a microcytic anemia, slight elevation in the INR, low ionized calcium, and albumin levels. Which of the following is the most likely cause for his diarrhea?