What is the most likely diagnosis for a 66-year-old female found unconscious with a cherry red appearance of the skin?

The presence of a cherry red appearance of the skin is a classic sign of carbon monoxide poisoning. Carbon monoxide (CO) binds to hemoglobin with a much higher affinity than oxygen, forming carboxyhemoglobin, which can prevent the blood from carrying adequate oxygen throughout the body. This binding alters the color of the blood, leading to the characteristic cherry red coloration of the skin and mucous membranes, as well as potentially causing significant neurological symptoms, including loss of consciousness.

In contrast, carbon dioxide poisoning typically does not present with any specific skin discoloration, and can instead lead to symptoms such as headache, dizziness, and confusion due to hypoxia or hypercapnia, which arise from excess carbon dioxide in the blood. Hydrogen sulfide exposure can cause other symptoms such as a rotten egg odor, but the skin color would not specifically appear cherry red. Severe dehydration is characterized by signs like dry skin, weakness, and alterations in mental status, rather than any distinct skin coloration changes.

In summary, recognizing the cherry red skin as indicative of carbon monoxide poisoning is crucial, as it directs the necessary immediate medical intervention to address this life-threatening condition.