A macrocytic anemia characteristically accompanies all of the following EXCEPT which one?

Macrocytic anemia is typically characterized by an increase in the size of red blood cells (macrocytes) and is commonly associated with deficiencies of certain vitamins, particularly vitamin B12 and folate. These deficiencies lead to impaired DNA synthesis, resulting in the production of larger than normal erythrocytes.

Vitamin B12 deficiency often occurs due to dietary insufficiencies or malabsorption conditions, leading to macrocytic anemia. Folate deficiency similarly affects DNA synthesis, causing similar macrocytic changes in red blood cells. Additionally, a fish tapeworm infection can lead to vitamin B12 deficiency as the parasite absorbs this essential nutrient, resulting in macrocytic anemia.

In contrast, iron deficiency anemia results in microcytic anemia, where red blood cells are smaller than normal due to inadequate hemoglobin production. This condition does not cause macrocytic changes, making it the outlier among the listed conditions. Therefore, the correct response to the question regarding which condition does not characteristically accompany macrocytic anemia is iron deficiency.