The nail apparatus has a limited number of ways it can express pathology. A basic understanding of the normal anatomy and physiology of this complex skin appendage is necessary to determine the site of pathology, formulate a differential diagnosis, and determine the appropriate workup. A more in-depth understanding of anatomy is absolutely crucial when performing diagnostic and therapeutic surgical procedures. Our understanding of nail unit biology is small in comparison with skin and hair biology, probably because of the difficult access to this organ. However, during the past decade, there have been many important advancements in our knowledge that are vital to an improved understanding of inflammatory, infectious, and neoplastic disease of the nail apparatus. This article is an update of nail anatomy and biology, as well as an update of many types of common nail disease. It is intended to serve as a practical guide for the dermatologist caring for patients with nail disorders.
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