This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Proteins and Amino Acids, Subject name: Biochemistry
Which of the following special amino acid is not formed by post-translational modification? 
Select and explain. 
A. Triodothyronine, B. Hydroxyproline, C. Hydroxylysine, D. Selenocysteine
D: Selenocysteine
Answer: D: Selenocysteine 
Explanation: Ans. D. SelenocysteineSelenocysteine is formed by cotranslational modification. All others are derived amino acids. Derived amino acids are formed by post-translational modification.