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: Unknown
[{'content': 'Cysteine is considered a nonessential amino acid only in the presence of dietary \nSelect and explain. \nA. Methionine, B. Serine, C. Folate, D. Phenylalanine', 'role': 'user'}, {'content': 'Answer: A \nExplanation: An amino acid is considered nonessential if it can be synthesized in humans, whereas it is considered essential if it must be obtained from the diet. Cysteine is derived from the essential amino acid methionine; therefore, cysteine can be produced in the body only if adequate dietary methionine is available. The 10 essential amino acids are phenylalanine, valine, trypto\xadphan, threonine, isoleucine, methionine, histidine, ar\xadginine, leucine, and lysine.', 'role': 'assistant'}]