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': 'Cyanosis does not occur in severe anemia because \nSelect and explain. \nA. Hypoxia stimulates erythropoietin secretion, B. Oxygen carrying capacity of available Hb is increased, C. Critical concentration of Hb required to produce cyanosis is reduced, D. Oxygen Dissociation Curve shifts to right', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Because reduced Hb is purple, a low aerial O2 saturation causes cyanosis. However, this is not a reliable sign of mild desaturation because its recognition depends on so many variables, such as lighting conditions and skin pigmentation. Because it is the amount of reduced Hb that is impoant, cyanosis is often marked when polycythemia is present but is difficult to detect in anemic patients.', 'role': 'assistant'}]