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: Medicine
Minimum concentration of reduced hemoglobin required for cyanosis is 
Select and explain. 
A. 1gm/dl, B. 3gm/dl, C. 5gm/dl, D. 7gm/dl
C: 5gm/dl
Answer: C: 5gm/dl 
Explanation: Cyanosis refers to a bluish discolouration of the skin and mucous membranes resulting from an increased quantity of reduced hemoglobin(I.e.,deoxygenated hemoglobin)or of hemoglobin derivatives(eg.,methemoglobin or sulfhemoglobin)in the small blood vessels of those tissues.Cyanosis becomes apparent when the concentration of reduced hemoglobin in capillary blood exceeds 5gm/dL. Ref:Harrison' s medicine-18th edition page no:288.