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
Human beings, during synthesis of fatty acids cannot introduce double bonds beyond the position: 
Select and explain. 
A. 4, B. 7, C. 9, D. 11
C: 9
Answer: C: 9 
Explanation: Higher animals have d4, d5, d6, and d9 desaturases but cannot inse new double bonds beyond the 9 position of fatty acids. Thus, the essential fatty acids linoleic (6) and -linolenic (3) must be obtained from the diet. Ref: Harper 28thedition, chapter 23.