Cyanosis is caused by

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%)