This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Orthopedics, Subject name: Surgery
[{'content': 'A rugby player fell onto his shoulder some weeks ago. He now presents with a painful lump about 4 cm in diameter on the top of his shoulder. What has he injured? \nSelect and explain. \nA. Common extensor origin, B. The acromioclavicular joint, C. The head of the humerus, D. The sternoclavicular joint', 'role': 'user'}, {'content': 'Answer: B \nExplanation: The acromioclavicular joint is typically injured by a fall on to the point of the shoulder. The result is inflammation then early aggressive arthritis which produces a painful lump just where the shoulder straps of a rucksack press on to the shoulder. The only possible treatment is to excise the joint, but this may not cure the pain.', 'role': 'assistant'}]