An 18-year-old man, of Italian extraction, is found to have a hypochromic microcytic anemia of 10 g/dL. In addition, there are a fair number of anisocytosis, poikilocytosis, and target cells seen on the blood film. The WBC is 9500/mL, the platelet count is 240,000/mL, and the reticulocyte count is 7%. The spleen is palpated 5 cm below the left costal margin. Which of the following is the most likely diagnosis?