This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Death and postmortem changes, Subject name: Forensic Medicine
[{'content': 'CSF should be collected within \nSelect and explain. \nA. 4-6 hours after death, B. 12-16 hours after death, C. 16-20 hours after death, D. 20-24 hours after death', 'role': 'user'}, {'content': "Answer: A \nExplanation: Cistern fluid is examined. The amount of potassium increases at a constant rate in relation to the temperature of the body during the first 20 hours. The concentrations of sodium,calcium and magnesium have no obvious relationship to time since death. Ref: K.S.Narayan Reddy's Synopsis of Forensic Medicine and Toxicology, 29th edition, Chapter 7, page-98.", 'role': 'assistant'}]