This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Psychiatry
[{'content': 'Delirium is distinguished from dementia by the presence of: \nSelect and explain. \nA. Impaired judgment, B. Impaired memory, C. Clouding of consciousness, D. Thought disorder', 'role': 'user'}, {'content': 'Answer: C \nExplanation: The hallmark of delirium is fluctuation in level of consciousness. Periods of lucency may be interspersed with periods of marked unresponsiveness. Another distinguishing feature is that the onset of delirium usually occurs within hours or days, whereas the onset of dementia may be insidious throughout a period of weeks to months. Ref: Johnston B., Yaffe K. (2008). Chapter 27. Dementia & Delirium. In M.D. Feldman, J.F. Christensen (Eds), Behavioral Medicine: A Guide for Clinical Practice, 3e.', 'role': 'assistant'}]