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: Medicine
The minimum concentration of reduced hemoglobin required for cyanosis is 
Select and explain. 
A. 1gm/dl., B. 3 gm/d, C. 7 gm/d, D. 5 gm/d
D: 5 gm/d
Answer: D: 5 gm/d 
Explanation: Cyanosis refers to a bluish colour of the skin and mucous membranes resulting from an increased quantity of reduced hemoglobin, or of hemoglobin derivatives, in the small blood vessels of those areas. It is usually most marked in the lips, nail beds, ears, and malar eminences.
The degree of cyanosis is modified by the colour of the cutaneous pigment and the thickness of the skin, as well as by the state of the cutaneous capillaries.
In some instances, central cyanosis can be detected reliably when the SaO2 has fallen to 85%; in others, particularly in dark-skinned persons, it may not be detected until it has declined to 75%.
The increase in the quantity of reduced hemoglobin in the mucocutaneous vessels that produces cyanosis may be brought about either by an increase in the quantity of venous blood as a result of dilation of the venules and venous ends of the capillaries or by a reduction in the Sa in the capillary blood. In sulhaemoglobmemia cyanosis occurs when its level exceeds 0.5gm/dl A methemogiobm level of 15% (i.e.,1.5gm/dl) 3 is associated with visible cyanosis-Nelson