A 34-yeаr-оld mаn with schizоphreniа has cоmplained of chronic fatigue for the last six months. He has a good appetite. However, he has refused to eat vegetables for the past year because "voices" tell him that vegetables are posioned. His physical and neurological exmaination are normal. His hemoglobin is 9.1 g/dl. His leukocyte count is 10,000/ mm3 and his mean red blood cell volume is 122. What is the most likely cause of his anemia and associated fatigue?