This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of lipid, Subject name: Biochemistry
The enzyme ^9 desaturase catalyzes the conversion of stearic acid into? 
Select and explain. 
A. Linolenic acid, B. Linoleic acid, C. Oleic acid, D. All of the above
C: Oleic acid
Answer: C: Oleic acid 
Explanation: The enzyme ^9 desaturase, in endoplasmic reticulum, catalyzes conversion of stearic acid to oleic acid, which is a MUFA. ^9 desaturase is a mixed-function oxidase, requiring NADPH and O2. Humans cannot introduce double bonds beyond ^9 position (between C10 and terminal methyl group). Hence, linoleic (D9,12 ) and linolenic acid (D 9,12,15) are considered essential fatty acids. Reference: Harper's Illustrated Biochemistry 30th Edition Pg 239