This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Structure and function of protein, Subject name: Biochemistry
Which of the following can be a homologous substitution for valine in hemoglobin? 
Select and explain. 
A. Isoleucine, B. Glutamic acid, C. Phenylalanine, D. Lysine
B: Glutamic acid
Answer: B: Glutamic acid 
Explanation: In Many examples of Point Mutations The classic example is sickle-cell disease, which is caused by mutation of a single base out of the 3 x 109 in the genome, a T-to-A DNA substitution, which in turn results in an A-to-U change in the mRNA corresponding to the sixth codon of the b-globin gene. The altered codon specifies a different amino acid (valine rather than glutamic acid), and this causes a structural abnormality of the b-globin molecule leading to hemoglobin aggregation and red blood cell "sickling." Ref: Harper's Illustrated Biochemistry, 30E (2015) Pg no 460.