This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of protein and amino acid, Subject name: Biochemistry
Selenocysteine is derivative of which amino acid? 
Select and explain. 
A. Cysteine, B. Alanine, C. Arginine, D. Glycine
A: Cysteine
Answer: A: Cysteine 
Explanation: Selenocysteine is the 21st amino acid.its biosynthesis is by replacing oxygen from serine by selenium.cysteine is formed from serine (reversible). hence selenocysteine is formed from cysteineRef: DM Vasudevan - Textbook of Biochemistry, 7th edition, page no: 215