Final answer:
The one-year-old girl with a hemoglobin level of 9 g/dl and a mean cell volume of 74, who is drinking more than 32 ounces of milk per day, is most likely diagnosed with Iron deficiency anemia.
Step-by-step explanation:
The most likely diagnosis for the one-year-old girl with a hemoglobin level of 9 g/dl and a mean cell volume of 74 is Iron deficiency anemia.
It is important to understand that iron deficiency anemia is quite prevalent among young children, especially those who may not be getting adequate iron in their diet. In this case, the child consuming more than 32 ounces of whole milk daily may be displacing intake of iron-rich foods, leading to anemia.
The presentation of anemia with a hemoglobin content of 5 to 9 g/dl, along with the fact that the child's mean cell volume (MCV) of 74 indicates microcytic anemia, supports this diagnosis. This is distinguished from β-thalassemia major which usually presents with much lower hemoglobin levels (about 2-3 g/dl) and a different clinical picture, as well as hereditary spherocytosis and lead poisoning, which also have distinctive clinical and laboratory features.