This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiovascular system, Subject name: Physiology
Cyanosis is caused by 
Select and explain. 
A. Reduced Hb above 7.5 g/dl, B. Met hb above 1.5 g/dl, C. Sulph. Hb above 0.5 g/dl, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: Cyanosis or blue discoloration of the mucous membrane is an index of hypoxia or decreased oxygen at the tissue level. It may occur in stagnant hypoxia, hypoxic hypoxia methemoglobinemia may be treated by reducing Fe by methylene blue. Severe anemic hypoxia <5 gm% can never result in cyanosis (reduced Hb >5 gm%)