A 4-month-old infant is anemic with a hemoglobin level of 9.5 g/dL and MCV 77 fL. The baby looks well, the height and weight growth parameters are normal, and she is breast-feeding well. Which of the following is the most likely cause for her anemia?

Iron deficiency is the most common cause of anemia in infancy. Sixty percent of body iron concentration at birth is contained in circulating Hb. Milk is a poor source of iron, so the most common cause of iron deficiency in infancy is prolonged breast or bottle feeding. Cereals are high in iron content. Hemolysis and folate deficiency as the cause of this baby's anemia would result in a normocytic or macrocytic anemia, and it is unlikely malabsorption is the cause since the baby is gaining weight and doing well clinically.