A teenage girl is pale, tired, and craves eating chalk (pica). This is iron deficiency anemia — the commonest nutritional deficiency worldwide: heavy periods + poor intake drain iron, hemoglobin falls, red cells turn microcytic (small) and hypochromic (pale). Ferritin (storage iron) is low — the diagnostic key distinguishing it from thalassemia. Pica (craving chalk, clay, ice) is the classic bizarre clue. Oral iron for 3 months rebuilds stores — but first find the blood loss: in adults, iron deficiency means search for GI bleeding.