A 25-year-old woman has a 3-year history of arthralgias. Physical examination shows no joint deformity, but she appears pale. Laboratory studies show a total RBC count of 4.7 million/ mm3, hemoglobin of 12.5 g/dL, hematocrit of 37.1%, platelet count of 217,000/ mm3, and WBC count of 5890/ mm3. The peripheral blood smear shows hypochromic and microcytic RBCs. Total serum iron and ferritin levels are normal. Hemoglobin electrophoresis shows 93% hemoglobin A1 with elevated hemoglobin A2 level of 5.8% and hemoglobin F level of 1.2%. What is the most likely diagnosis?