A 47-year-old man presents with a 6-week history of increasing fatigue and dark-colored stools. Complete blood count shows hemoglobin of 8.6 g/dL and microcytic, hypochromic RBCs. Upper gastrointestinal endoscopy reveals a peptic ulcer along the lesser curvature of the stomach. This patient's anemia is most likely caused by deficiency of which of the following?