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: Biochemistry
Purpose of Gene therapy - 
Select and explain. 
A. Replacement of abnormal gene by normal gene, B. Replacement of normal gene by abnormal gene, C. Knock out of abnormal gene, D. Introduction of viral gene
A: Replacement of abnormal gene by normal gene
Answer: A: Replacement of abnormal gene by normal gene 
Explanation: The most common approach is to insert a normal gene into non-specific location within the genome to replace a non­functional gene (abnormal gene).