A 4-year-old boy has a chronic microcytic anemia, but no other symptoms. His physical examination is normal except for splenomegaly. His condition is due to decreased alpha-chain production, which leads to four beta-chain tetramer formation (HbH).For the above patient with a hemoglobin abnormality, select the most likely diagnosis.
beta-thalassemia major HbH disease sickle cell disease HbC disease