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
Which one of the following amino acids is purely ketogenic? 
Select and explain. 
A. Proline, B. Phenylalanine, C. Isoleucine, D. Leucine
D: Leucine
Answer: D: Leucine 
Explanation: None