A patient develops a persistent macrocytic anemia. Serum folate levels are normal, but serum vitamin B12 levels are low. Oral vitamin absorption studies demonstrate that the patient is unable to absorb vitamin B12 in adequate amounts. Cancer of which of the following organs is most strongly associated with this patient's condition?

Megaloblastic anemia with poor vitamin B12 absorption suggests pernicious anemia. In this condition, an autoimmune attack on gastric parietal cells leads to atrophic gastritis with deficient synthesis of the intrinsic factor needed for vitamin B12 absorption. Vitamin B12 is required for DNA synthesis in blood cell precursors; deficiency produces megaloblastic anemia. Chronic atrophic gastritis predisposes an individual to the development of gastric carcinoma. Predisposing factors for colon cancer include familial polyposis syndromes, inflammatory bowel disease, isolated adenomatous polyps, and diet. Duodenal and ileal cancers are uncommon. Predisposing factors for esophageal cancer include Barrett's esophagus and Plummer- Vinson syndrome. Ref: Bender D.A. (2011). Chapter 44. Micronutrients: Vitamins & Minerals. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.