"C" is right
The Schilling test is performed to identify the cause of vitamin B12 deficiency. In the first step of the test, an oral dose of radiolabeled vitamin B12 is given concomitantly with an intramuscular injection of non-radiolabeled vitamin B12. Urine is collected for the next 24 to 48 hours, and the excretion of radioactive vitamin B12 is measured. High urinary excretion of radioactive cobalamin during this step is evidence of normal absorption of this vitamin and is diagnostic of dietary B12 deficiency. The purpose of concomitant parenteral administration of vitamin B12 is to ensure excretion of the radiolabeled form by creating a state of excess circulating vitamin B12. If the parenteral dose were not administered, all of the orally administered radioactive B12 would be taken up by the tissues in the case of dietary vitamin deficiency and little would be excreted in the urine.
Low urinary excretion of radioactive cobalamin rules out dietary deficiency and is suggestive of poor absorption of this vitamin. Poor absorption can result from lack of intrinsic factor (pernicious anemia) or from a malabsorption syndrome. Phase II of the Schilling test helps to differentiate between these two causes.
During phase II, radiolabeled B12 is administered concomitantly with intrinsic factor, and its urinary excretion is measured. If the cause of impaired absorption of cobalamin is lack of intrinsic factor, it will be corrected during this stage of testing and urinary excretion of radiolabeled vitamin B12 would increase. Low excretion of radiolabeled cobalamin after administration of intrinsic factor suggests another cause of poor cobalamin absorption. Causes of intestinal malabsorption of B12 include pancreatic insufficiency, intestinal bacterial overgrowth or ileal disease (Choice C).
(Choice B) Atrophic gastritis is associated with pernicious anemia. Autoimmune destruction of parietal cells leads to decreased secretion of intrinsic factor.
(Choice D) Myelodysplasia is a disorder of maturation of hematopoietic cells. It is associated with peripheral pancytopenia (anemia, neutropenia and thrombocytopenia) along with a hypercellular bone marrow. Myelodysplasia is a premalignant condition and does not cause vitamin B12 deficiency.
(Choice E) Chronic low-volume GI bleeding results in hypochromic microcytic iron-deficiency anemia.
