Which of the following conditions is commonly NOT associated with microcytic anemia?

Microcytic anemia is characterized by red blood cells that are smaller than normal, often due to decreased hemoglobin content. The common causes of microcytic anemia include iron deficiency, thalassemia, and anemia of chronic disease.

Iron deficiency is the most prevalent cause of microcytic anemia, as it leads to inadequate hemoglobin synthesis. Thalassemia, a genetic disorder affecting hemoglobin production, also results in microcytic red blood cells. Anemia of chronic disease can present with microcytic or normocytic red blood cells, but it is often associated with iron sequestration and inflammation, which can reduce the availability of iron for hemoglobin synthesis, leading to a microcytic presentation.

In contrast, vitamin B12 deficiency is typically associated with macrocytic anemia, where the red blood cells are larger than normal due to impaired DNA synthesis leading to ineffective erythropoiesis. This enlargement of red blood cells is distinct from the microcytic nature of the other conditions listed, and thus vitamin B12 deficiency is commonly not associated with microcytic anemia.