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: Surgery
[{'content': 'Which of the following is the most common location of branchial cyst? \nSelect and explain. \nA. Upper ?rd of anterior border of SCM, B. Upper ?rd of posterior border of SCM, C. Lower ?rd of posterior border of SCM, D. Lower ?rd of anterior border of SCM', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Brachial cyst is most frequently located at the anterior border of sternocleidomastoid where the upper third meets the middle third. It usually arise from the failure of obliteration of pharyngobranchial ducts during fetal development. It appear as a tender inflammatory mass.', 'role': 'assistant'}]