This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Miscellaneous (Bio-Chemistry), Subject name: Biochemistry
Sulphur containing amino acid is 
Select and explain. 
A. Cysteine, B. Lysine, C. Arginine, D. Phenylalanine
A: Cysteine
Answer: A: Cysteine 
Explanation: (A) Cysteine # Sulfur-containing amino acids (cysteine & methionine) are generally considered to be nonpolar and hydrophobic.> In fact, methionine is one of the most hydrophobic amino acids and is almost always found on the interior of proteins.> Cysteine on the other hand does ionize to yield the thiolate anion.