Which of the following defines methemoglobinemia?

Methemoglobinemia is characterized by an increase in methemoglobin levels in the blood, which decreases the oxygen-carrying capacity of hemoglobin. In this condition, the iron in hemoglobin is oxidized from the ferrous (Fe2+) state to the ferric (Fe3+) state, leading to a reduced ability to bind and transport oxygen effectively. This results in symptoms such as cyanosis (a bluish discoloration of the skin) and reduced oxygen delivery to tissues, making it a significant clinical concern.

The other options do not accurately describe methemoglobinemia. Increased red blood cell production relates to conditions such as polycythemia, whereas enhancements of hemoglobin function refer to improved oxygen affinity or biochemical alterations unrelated to methemoglobin levels. An elevation of platelet count in blood pertains to thrombocytosis, which is also not related to the metabolic state of hemoglobin or oxygen transport.