The "hair on end" appearance on skull radiographs results from marrow hyperplasia due to increased erythropoiesis in response to chronic anemia. This feature is prominent in hemolytic anemias like beta-thalassemia major, sickle cell anemia, and congenital dyserythropoietic anemia, where the bone marrow expands significantly. In contrast, iron deficiency anemia does not cause marrow expansion or trabecular thickening, as it is a hypoproliferative anemia with reduced red cell production, thus lacking the "hair on end" radiographic changes.

Key surgical point: The presence of "hair on end" is indicative of marrow hyperactivity, not seen in iron deficiency anemia.

Reference: Harrison's Principles of Internal Medicine, 20th Edition.