An 18-year-old pregnant woman receives no prenatal care, eats a diet containing mostly carbohydrates and fats, and does not take prenatal vitamins with iron. She feels increasingly tired and weak during the third trimester. The infant is born at 35 weeks' gestation and is listless during the first week of life. Laboratory studies show markedly decreased serum ferritin levels in the infant and the mother. Which of the following findings from a nutritional deficiency is most likely to be present in both the infant and the mother?