Conditions of the human integumentary system constitute a broad spectrum of diseases, also known as dermatoses , as well as many nonpathologic states (like, in certain circumstances, melanonychia and racquet nails ).   While only a small number of skin diseases account for most visits to the physician , thousands of skin conditions have been described.  Classification of these conditions often presents many nosological challenges, since underlying etiologies and pathogenetics are often not known.   Therefore, most current textbooks present a classification based on location (for example, conditions of the mucous membrane ), morphology ( chronic blistering conditions ), etiology ( skin conditions resulting from physical factors ), and so on.   Clinically, the diagnosis of any particular skin condition is made by gathering pertinent information regarding the presenting skin lesion (s), including the location (such as arms, head, legs), symptoms ( pruritus , pain), duration (acute or chronic), arrangement (solitary, generalized, annular, linear), morphology ( macules , papules , vesicles ), and color (red, blue, brown, black, white, yellow).  Diagnosis of many conditions often also requires a skin biopsy which yields histologic information   that can be correlated with the clinical presentation and any laboratory data.