This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
In human body, methionine is synthesized from ? 
Select and explain. 
A. Cysteine, B. Proline, C. Threonine, D. None
D: None
Answer: D: None 
Explanation: Ans. is 'd' i.e., None Methionine is an essential amino acid, cannot be synthesized in body.