<unk>  is usually drug induced, with drugs associated with it including sulphonamides, such as sulfasalazine or sumatriptan. Another possible cause is occupational exposure to sulfur compounds. It can also be caused by phenazopyridine.
Sulfhemoglobinemia
Methemoglobinemia
Organic acidemia
Cyanide poisoning
Hyperaemia
Isovaleric acidemia
Propionic acidemia
Algaemia