In a patient with megaloblastic anemia, Schilling test shows B12 detected in urine only after intrinsic factor (IF) is given. What is the presumptive diagnosis?

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This question hinges on how the Schilling test separates intrinsic factor–dependent B12 absorption from other causes of B12 deficiency. In this test, radiolabeled B12 is given, and how much ends up in the urine reflects how well B12 was absorbed. If absorption is poor without intrinsic factor but normalizes when intrinsic factor is added, the defect is in intrinsic factor availability, not in the ileum or dietary intake.

If B12 is detected in the urine only after intrinsic factor is provided, it means the patient’s B12 absorption depends on intrinsic factor. The most likely cause is loss or absence of intrinsic factor due to pernicious anemia, where autoimmune destruction of gastric parietal cells or antibodies against intrinsic factor lead to IF deficiency and consequent B12 malabsorption. This pattern clearly points to pernicious anemia as the presumptive diagnosis.

Folate deficiency also causes megaloblastic anemia but doesn’t hinge on intrinsic factor for B12 absorption, so IF would not be required to see B12 uptake in that scenario. B12 deficiency from malnutrition would typically not show a dependence on IF for absorption. Aplastic anemia is a marrow failure disorder and does not involve B12 absorption or IF status.