This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory System, Subject name: Physiology
[{'content': 'Concentration of methemoglobin to cause cyanosis- \nSelect and explain. \nA. 5 gm/dl, B. 2 gm/dl, C. 1.5 gm/dl, D. 12 gm/dl', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. (c) 1.5gm/dl(Ref: Ganong, 25th ed/p.647)The level of methemoglobin producing cyanosis is 1.5gm/dlIf deoxyhemoglobin (reduced Hb) is 5.0g/dL or greater, cyanosis appears', 'role': 'assistant'}]