A 70-year-old man with no prior surgical history presents with a tingling sensation of the extremities, weakness, and an enlarged tongue that is sore and smooth. Preliminary laboratory studies are negative for antinuclear antibodies and antimitochondrial antibodies, and peripheral blood smear reveals a macrocytic megaloblastic anemia. When the patient is given an oral dose of radiolabeled cyanocobalamin, none of that compound is detected in his urine. What is the function of the protein that is most likely defi cient and/or affected in this disease process?
(A) Binds free vitamin B12 in ileal cells for transport through the bloodstream
(B) Binds free vitamin B12 in the stomach
© Complex with free vitamin B12 in the small intestine and then binds to receptors on the wall of the ileum
(D) Splits R-protein/vitamin B12 complexes in the duodenum
(E) Splits vitamin B12 from its exogenous ingested protein-bound form