This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Proteins and Amino Acids, Subject name: Biochemistry
On consumption of which aminoacid, requirement of methionine is reduced because of sparing effect? 
Select and explain. 
A. Homocysteine, B. Cysteine, C. Lysine, D. Arginine
B: Cysteine
Answer: B: Cysteine 
Explanation: Ans. is 'b' i.e., Cysteine"Adequate dietary source of cysteine 'spares' methionine".* Methionine is an essential amino acid.* Methionine is required for synthesis of cysteine (thus cysteine is non-essential amino acid)* Thus, cysteine can spare the requirment of methionine in diet.