This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory system, Subject name: Medicine
[{'content': 'Shifting dullness is the characteristic sign of \nSelect and explain. \nA. Hydropneumothorax, B. Pneumothorax, C. Consolidation, D. Collapse', 'role': 'user'}, {'content': "Answer: A \nExplanation: Hydropneumothorax is collection of both fluid(lower pa) and air(upper pa) in the pleural cavity.The hallmark diagnosis in hydropneumothorax is 3 'S':Shifting dullness(due to the presence of free fluid & air within the pleural cavity,dullness of fluid shifts from one place to other within chest with the change of posture of the patient),Succussion splash,and Straight fluid level(upper limit of dullness is horizontal).", 'role': 'assistant'}]