This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nervous system, Subject name: Radiology
[{'content': 'Dense middle cerebral aery (MCA) Sign in brain is seen in \nSelect and explain. \nA. Acute infarct, B. Subacute infarct, C. Subacute haemorrhage, D. Chronic infarct', 'role': 'user'}, {'content': 'Answer: A \nExplanation: The hyperdense MCA sign refers to the focally increased density of the middle cerebral aery on CT and is a direct visualization of thromboembolic material within the lumen. It is thus the earliest visible sign of MCA infarction as it is seen within 90 minutes after the event - acute infarct. It is the longitudinal equivalent of the MCA dot sign and hyperdense basilar tip sign.', 'role': 'assistant'}]