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
In maple syrup disease, all of the following amino acids are excreted in urine, EXCEPT: 
Select and explain. 
A. Valine, B. Leucine, C. Isoleucine, D. Phenylalanine
D: Phenylalanine
Answer: D: Phenylalanine 
Explanation: The biochemical defect in maple syrup urine disease (branched-chain ketonuria, or MSUD) involves the alpha-keto acid decarboxylase complex. Plasma and urinary levels of leucine, isoleucine, valine, and their alpha-keto acids and alpha-hydroxy acids (reduced alpha-keto acids) are elevated, but the urinary keto acids derive principally from leucine. Signs and symptoms of MSUD include often fatal ketoacidosis, neurological derangements, mental retardation, and a maple syrup odor of urine. Ref: Rodwell V.W. (2011). Chapter 29. Catabolism of the Carbon Skeletons of Amino Acids. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.