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
The primary defect which leads to sickle cell anemia is 
Select and explain. 
A. 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.
B: Replacement of glutamate by valine in B-chain of HbA
Answer: B: Replacement of glutamate by valine in B-chain of HbA 
Explanation: None