A 29-year-old woman was found to have Hb of 7.8mg/dl with a reticulocyte count of 0.8%. The peripheral blood smear showed microcytic hypochromic anemia. On HPLC, Hb A2 and Hb F was 2.4% and 1.3% respectively. The serum iron and TIBC were 15mg/dl and 420 micro g/dl respectively. The most likely cause of anemia is: