This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Classification and metabolism of amino acids, Subject name: Biochemistry
All are true about selenocystiene EXCEPT: 
Select and explain. 
A. Considered as 21stAmino acid, B. Coded by UGA, C. Occurs in glutathione peroxidase, D. Made of cysteine and methionine
D: Made of cysteine and methionine
Answer: D: Made of cysteine and methionine 
Explanation: Selenocysteine is found in approximately 25 human proteins. Enzymes which use selenocysteine at their active sites are known as Selenoproteins e.g. 1. Glutathione peroxidase 2. Thioredoxinreductase 3. Iodothyroninedeiodinase Precursor amino acid for selenocysteine is serine. (not cysteine or methionine) Coded by UGA codon