Which of the following types of anemia would result in an increased Mean Corpuscular Volume (MCV)?

The increase in Mean Corpuscular Volume (MCV) is associated with macrocytic anemias, which are typically characterized by the presence of larger-than-normal red blood cells. In the context of the question, vitamin B12 deficiency results in a macrocytic anemia due to the impaired synthesis of DNA, which leads to ineffective hematopoiesis and the production of larger erythrocytes.

Vitamin B12 is crucial for proper DNA synthesis within red blood cell precursors, and a deficiency can cause a slowed process of cell division while allowing cell growth to continue, hence producing larger cells. This condition often leads to an increased MCV.

While folic acid deficiency can also lead to macrocytic anemia and increased MCV, the question specifically highlights vitamin B12 deficiency as a clear example. Iron deficiency anemia and aplastic anemia do not typically result in elevated MCV; iron deficiency anemia usually presents with microcytic red blood cells and lower MCV, while aplastic anemia is associated with a wide variety of cell sizes but not specifically increased MCV. Thus, the correct answer emphasizes the well-established link between vitamin B12 deficiency and increased MCV in the setting of macrocytic anemia.