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
Cysteine is synthesized from 
Select and explain. 
A. Methionine, B. Arginine, C. Histidine, D. Lysine
A: Methionine
Answer: A: Methionine 
Explanation: Methionine is first conveed to active methionine and then it is conveed to cysteine on combining with serine.Ref: DM Vasudevan - Textbook of Biochemistry, 7th edition, page no: 217, Fig 16.14, 16.15.