Which condition is NOT a differential diagnosis for normal MCV?

Normal mean corpuscular volume (MCV) indicates that the average size of red blood cells is within the normal range. In this context, vitamin B12 deficiency typically leads to a macrocytic anemia, which is characterized by an increased MCV due to the production of larger-than-normal red blood cells. This condition arises because vitamin B12 is essential for proper red blood cell formation in the bone marrow.

On the other hand, anemia of chronic disease is often associated with a normal or low MCV due to the underlying inflammatory state affecting red blood cell production. Sickle cell disease, while it can result in a range of MCV values, commonly presents with normal MCV in patients without any other concurrent anemias. Renal failure can also lead to anemia with a normal MCV, primarily as a result of decreased erythropoietin production affecting red blood cell production in a manner that doesn't alter the red blood cell size significantly.

Therefore, among the conditions listed, the presence of normal MCV in vitamin B12 deficiency stands out as an inconsistency, confirming it as the condition that does not fit within the differential diagnoses for normal MCV.