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: Physiology
Central cyanosis is seen if 
Select and explain. 
A. Methemoglobin 0.5 gm/dl, B. O2 saturation < 85%, C. O2 saturation < 94%, D. Hb – 4 gm%
B: O2 saturation < 85%
Answer: B: O2 saturation < 85% 
Explanation: Oxygen saturation should be less than 85% for central cyanosis to occur. Deoxyhemoglobin level should be at least 5gm%.