This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Fatty acid and synthesis, Subject name: Biochemistry
Conversion of stearic acid to oleic acid is catalysed by: 
Select and explain. 
A. D3desaturase, B. D6desaturase, C. D9desaturase, D. D12desaturase
C: D9desaturase
Answer: C: D9desaturase 
Explanation: Desaturase enzymes can't introduce double bonds beyond the D9 position. This is why ceain fatty acids are essential and must be taken in diet.