This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
[{'content': 'The primary defect which leads to sickle cell anemia is \nSelect and explain. \nA. An abnormality in porphyrin part of hemoglobin, B. Replacement of glutamate by valine in B-chain of HbA, C. A nonsence mutation in the B-chain of HbA, D. Substitution of valine by glutamate in the A-chain of HbA.', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]