This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Amino Acid Metabolism, Subject name: Biochemistry
Putrescene - a polyamine is formed by decarboxylation of: 
Select and explain. 
A. Arginine, B. Ornithine, C. Citrulline, D. Adenine
B: Ornithine
Answer: B: Ornithine 
Explanation: Ans: B (Ornithine)Ref: Harper's Illustrated Biochemistry,28th editionExplanation:Polyamines include putrescine, spermidine, spermine.Ornithine is a major precursor for synthesis of polyamines.Conversion of ornithine to putrescine is catalyzed by Ornithine Decarboxylase. (See figure)