This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Blood, Subject name: Medicine
[{'content': '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. \nSelect and explain. \nA. beta-thalassemia major, B. HbH disease, C. sickle cell disease, D. HbC disease', 'role': 'user'}, {'content': 'Answer: B \nExplanation: HbH (four beta-chains) most commonly results from the compound heterozygous state for alpha thalassemia (-/-a). The phenotypic expression of the disease is quite variable.', 'role': 'assistant'}]