During the workup of a 52-year-old man with macrocytic anemia (MCV 108 fL, Hb 7.2 g/dL), the following investigations are performed: serum B12 level 180 pg/mL (low-normal), serum folate 2.1 ng/mL (low), methylmalonic acid (MMA) elevated, homocysteine elevated, and bone marrow shows hypersegmented neutrophils. All of the following support a diagnosis of vitamin B12 deficiency EXCEPT: