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
The amino acid at the reducing end of glutathione: 
Select and explain. 
A. Glycine, B. Cysteine, C. Lysine, D. Glutamate
B: Cysteine
Answer: B: Cysteine 
Explanation: None