Q. A 72-year-old woman comes to her primary care physician with complaints of chronic fatigue and weakness. She also reports a strange "tingling" sensation in her feet and hands that has persisted over the last few months. The patient also mentions that she recently started feeling dizzy when she walks and has trouble coordinating her movements. On physical examination, the physician notes mild pallor and scleral icterus. Neurological examination reveals a loss of proprioception and vibratory sense, especially in the lower extremities. Labs show macrocytic anemia. Further workup is performed to confirm the diagnosis. Given the patient's presentation, which of the following conditions is most likely responsible for her symptoms?