GI System MCAT Practice Question
A 68-year-old woman presents with progressive fatigue, paresthesias, and macrocytic anemia. Laboratory studies show a hemoglobin of 8.2 g/dL, MCV of 108 fL, and a serum B12 level of 180 pg/mL (normal 200-900). Upper endoscopy reveals chronic atrophic gastritis with intestinal metaplasia. Intrinsic factor and parietal cell antibodies are positive. She is started on oral cyanocobalamin 1000 mcg daily, but repeat B12 levels after 8 weeks remain low at 165 pg/mL. Which of the following best explains the inadequacy of oral supplementation in this patient?
Answer choices
- AOral B12 undergoes oxidative degradation in the acidic environment of the proximal small intestine
- BAchlorhydria prevents the dissociation of dietary B12 from food protein complexes in the stomach
- CWithout intrinsic factor, B12 cannot be recognized by specific receptors on the terminal ileal mucosa required for active transportCorrect answer
- DIntestinal metaplasia reduces the surface area available for passive diffusion of B12 across the ileal epithelium
- EAtrophic gastritis causes bacterial overgrowth that competitively consumes oral B12 before absorption
- FParietal cell destruction eliminates gastric proteases required to cleave B12 from protein-bound forms
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