A 48-year-old woman with a 5-year history of GERD is on omeprazole 40 mg daily. She presents with fatigue, glossitis, and paresthesias in both feet. Serum B₁₂ is 180 pg/mL (normal >200), and intrinsic factor antibodies are negative. Gastric parietal cell antibodies are also negative. What is the most likely mechanism of B₁₂ deficiency in this patient?

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