This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Neuroradiology, Subject name: Radiology
[{'content': "Absent swallow tail sign is seen in \nSelect and explain. \nA. Huntington's chorea, B. Alzheimer's disease, C. Parkinsonism, D. Progressive supranuclear palsy", 'role': 'user'}, {'content': 'Answer: C \nExplanation: SWALLOW TAIL SIGN: The Swallow tail sign describes the normal axial imaging appearance of nigrosome-1 within the substantia nigra on high resolution T2*/SWI weighted MRI. Absence of the sign (absent swallow tail sign) is repoed to have a diagnostic accuracy of greater than 90% for Parkinson disease. Figure showing normal swallow tail. Figure showing absent swallow tail Figure showing absent swallow tail sign in Parkinsonism.', 'role': 'assistant'}]