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
Adenosine receptor stability is because of extensive disulfide bonds formed between: 
Select and explain. 
A. Cysteine, B. Methionine, C. Arginine, D. Alanine
A: Cysteine
Answer: A: Cysteine 
Explanation: Disulfide bonds can only be formed between one amino acid i.e. cysteine, as it has sulfhydryl group (-SH). Two cysteine gets joined to form cystine (having disulfide bond) (see fig). It a strong covalent bond.