Answer: c) Absorption of cobalamin occurs by active process in ileum onlyCOBALAMIN* In nature, the vitamin is mainly in the 2-deoxyadenosyl (ado) form, which is located in mitochondria. It is the cofactor for the enzyme methylmalonyl coenzyme A (CoA) mutase.* The other major natural cobalamin is methylcobalamin, the form in human plasma and in cell cytoplasm. It is the cofactor for methionine synthase.* Two mechanisms exist for cobalamin absorption.o Passive: occurring equally through buccal, duodenal, and ileal mucosa; it is rapid but extremely inefficient, with <1% of an oral dose being absorbed by this process,o The normal physiologic mechanism is active; it occurs through the ileum and is efficient for small (a few micrograms) oral doses of cobalamin# Mediated by gastric intrinsic factor (IF).# Dietary cobalamin is released from protein complexes by enzymes in the stomach, duodenum, and jejunum# It combines rapidly with a salivary glycoprotein known as haptocorrins (HCs).# In the intestine, the haptocorrin is digested by pancreatic trypsin and the cobalamin is transferred to IF.* IF (gene at chromosome 11q13) is produced in the gastric parietal cells.* The IF-cobalamin complex passes to the ileum, where IF attaches to a specific receptor (cubilin) on the microvillus membrane of the enterocytes.* Cubilin also is present in yolk sac and renal proximal tubular epithelium.* The cobalamin-IF complex enters the ileal cell, where IF is destroyed.* After a delay of about 6 h, cobalamin appears in portal blood attached to transcobalamin II.* Between 0.5 and 5 mg of cobalamin enter the bile each day.* This binds to IF, and a major portion of biliary cobalamin normally is reabsorbed together with cobalamin derived from sloughed intestinal cells.* Because of the appreciable amount of cobalamin undergoing enterohepatic circulation, cobalamin deficiency develops more rapidly in individuals who malabsorb cobalamin than it does in vegans, in whom reabsorption of biliary cobalamin is intact.