This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Misc. (R.B.C), Subject name: Pathology
There is no cyanosis in severe anemia because - 
Select and explain. 
A. Certain min. amount of reduced Hb should be present, B. In anemia, O2 saturation increases, C. Hypoxia stimulates erythropoietin production, D. O2 hemoglobin curve shifts to right
A: Certain min. amount of reduced Hb should be present
Answer: A: Certain min. amount of reduced Hb should be present 
Explanation: Ans is 'a' i.e. Certain min. amount of reduced Hb should be present o Cyanosis is a blue coloration of the skin and mucous membranes due to the presence of >5g/dl reduced hemoglobin in blood vessels near the skin surface. Now since in anemia the total amount of hemoglobin is decreased, the amount of reduced Hb to produce Cyanosis is not sufficient.