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
The conversion of a fatty acid to an active fatty acid is catalysed by which of the following enzymes? 
Select and explain. 
A. Acyl-CoA  synthetase, B. Enoyl CoA hydratase, C. Thiolase, D. Acyl CoA dehydrogenase
A: Acyl-CoA  synthetase
Answer: A: Acyl-CoA  synthetase 
Explanation: Fatty acids must first be converted to an active intermediate before they can be catabolized. This is the only step in the complete degradation of a fatty acid that requires energy from  ATP.  In  the  presence  of  ATP and coenzyme A, the enzyme acyl-CoA synthetase (thiokinase) catalyzes the conversion of a fatty acid (or FFA) to an “active fatty acid” or  acyl-CoA, using one high-energy phosphate and forming AMP and PPi . The PPi is hydrolyzed by inorganic pyrophosphatase with the loss of a further high-energy phosphate, ensuring that the overall reaction goes to completion. Acyl-CoA  synthetases  are  found  in  the  endoplasmic reticulum, peroxisomes, and inside and on the outer membrane of mitochondria.
HARPERS ILLUSTRATED BIOCHEMISTRY30th ed page no. 224