In the case of an anemic adult with an MCV of 125 fL and an RDW of 20%, which of the following could be part of the differential diagnosis?

The scenario describes an anemic adult with a mean corpuscular volume (MCV) of 125 fL and a red cell distribution width (RDW) of 20%. An MCV of 125 fL indicates macrocytic anemia, and the elevated RDW suggests a variation in red blood cell size, which is often seen in conditions where there's a significant lack of vitamin B12 or folate.

Vitamin B12 deficiency is associated with macrocytic anemia characterized by an increase in MCV, as well as a higher RDW due to the presence of both normal and large red blood cells as the bone marrow attempts to compensate for the deficiency. This makes vitamin B12 deficiency a strong candidate in the differential diagnosis given the presented MCV and RDW values.

In contrast, aplastic anemia typically presents with normocytic red blood cells and does not usually show a high RDW. Sickle cell anemia is primarily a normocytic anemia and is characterized by sickling of red cells rather than the macrocytic picture indicated here. Iron deficiency anemia typically presents as microcytic, with a low MCV, which does not align with the findings of increased MCV here.

Therefore, among the choices provided, vitamin B12 deficiency