A 73-year-old-woman presents to the clinic complaining of fatigue and feeling unwell. She notes the symptoms are worse on exertion, but denies any chest pain or shortness of breath. Her past medical history is significant for hypertension and type 2 diabetes. Both conditions are well controlled on her current medications. On physical examination, the vital signs are – blood pressure 135/80 mm Hg, pulse 72/min, and respiratory rate 10/min. The lungs are clear on auscultation and the heart sounds are normal. A CBC reveals a hemoglobin value of 9.5 g/dL and a MCV 105 mm3. Which of the following typically causes a macrocytic anemia?