This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: C.V.S, Subject name: Medicine
[{'content': "Hamman's sign is seen in- \nSelect and explain. \nA. Pneumomediastinum, B. Pericardial effusion, C. SVC obstruction, D. Thyrotoxicosis", 'role': 'user'}, {'content': "Answer: A \nExplanation: hamman sign is a feature of pnuemo mediastinum Hamman's sign (rarely, Hammond's sign or Hammond's crunch) is a crunching, rasping sound, synchronous with the heabeat, heard over the precordium in spontaneous mediastinal emphysema. It is felt to result from the hea beating against air-filled tissues. It is named after Johns Hopkins clinician Louis Hamman, M.D. Ref Davidson 23rd edition pg 440", 'role': 'assistant'}]