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: Surgery
[{'content': 'A patient brought to the emergency room after a road traffic accident shows cyanosis. Which of the following is TRUE regarding in this patient? \nSelect and explain. \nA. Cyanosis is a early s/o hypoxia, B. Cyanosis is a late s/o hypoxia, C. Absence of cyanosis indicates adequate airway, D. Absence of cyanosis indicates adequate tissue perfusion', 'role': 'user'}, {'content': "Answer: B \nExplanation: Cyanosis becomes apparent when the concentration of reduced hemoglobin in capillary blood exceeds 40 g/L (4 g/dL). Cyanosis is an extremely late sign of hypoxemia and may not occur at all if there has been extensive blood loss. Indeed, a patient has to have 5 grams of deoxygenated hemoglobin per 100 cc of blood for cyanosis to occur. Ref: Loscalzo J. (2012). Chapter 35. Hypoxia and Cyanosis. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds), Harrison's Principles of Internal Medicine, 18e.", 'role': 'assistant'}]