[Recognize rare diseases on the skin].

The correct interpretation of skin manifestations can facilitate the diagnosis of many rare systemic diseases. Such manifestations can be due to autoimmune diseases (e.g. dermatomyositis, systemic lupus erythematosus, systemic sclerosis and sarcoidosis) and metabolic diseases (e.g. Anderson-Fabry disease and porphyria cutanea tarda). Other cutaneous symptoms are of great importance because they are possible warning signs of occult diseases of internal organs. This is true for example for some diseases from the group of neutrophilic dermatoses, such as Sweet's syndrome and pyoderma gangraenosum.