Which disorder is characterized by slightly bluish/dark-purple skin discoloration, most easily seen in the nail beds and mucous membranes, due to an increased quantity of methemoglobin, hemoglobin not combined with oxygen in systemic blood.

Cyanosis

Explanation:

Cyanosis is purplish or bluish discolouration of skin or the mucous membranes. The presence of the cyanosis occurs due to the increased quantity of the deoxyhemoglobin and the haemoglobin is not combined with the oxygen. The tissues near the surface of the skin become to have low oxygen saturation. The first signs of the cyanosis can be seen on lips or fingers or finger nails.