A 29-year-old woman was found to have a hemoglobin of 7.8 g/dl, with a reticulocyte count of 0.8%. The peripheral blood smear showed microcytic, hypochromic anemia. Hemoglobin A2 and hemoglobin F levels were 2.4% and 1.3% respectively. The serum iron was low and the total iron binding capacity (TIBC) was raised at 420 mcg/dL. The most likely cause of anemia is: