A 29 year old woman was found to have a hemoglobin of 7.8 g/dl. with a reticulocyte count of 0.8%. The peripherial blood smear showed microcytic hypochromic anemia, Hemoglobin A2 and hemoglobin F levels were 2.4% and 1.3% respectively. The serum iron and the total iron binding capacity were 15 micro g/dl, and 420 micro g/dl, respectively. The most likely cause of anemia is: