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any disease or deformity of the nail. Congenital onychosis is caused by hereditary abnormalities of cornification. Acquired onychosis can be caused by functional disturbances of the bodily systems, for example, the endocrine and the nervous systems; by external influences, such as chemicals and mechanical injuries; and by infection of the nail plates by pathogenic fungi or microorganisms, including pyogenic cocci and the tubercle bacillus. Onychosis often results from skin diseases, including eczema and psoriasis. It is treated by eliminating the underlying cause.