The doctor should be skeptical because the blue color in the nail beds and lips means that the person has either lost a large amount of blood, or blood is just not reaching these areas of the body.
The doctor should be skeptical because carbon monoxide would cause harm to humans only if it were at extremely high concentrations in the air we breathe.
The doctor should be skeptical because carbon monoxide breaks apart into carbon and oxygen. The oxygen can then bind to the hemoglobin in the red blood cells.
The doctor should be skeptical because when carbon monoxide binds to hemoglobin, it produces the bright red color seen when oxygen binds to this molecule.