Median canaliform nail dystrophy is characterized by a longitudinal groove in the midline of the nail plate. The entire appearance resembles that of an inverse "fir-tree" pattern. It most commonly affects the nails of both the thumbs but it can also be unilateral or it can also affect other fingernails. It is usually idiopathic but sometimes it is caused due to micro trauma of the nail matrix and it has even been associated with isotretinoin use rarely. There's no specific treatment but general measures like clipping the nail short and wrapping it in a bandage to prevent further trauma and catching are useful. In case of an association with isotretinoin use, discontinuing the medication helps to treat it. Image source: dermnetnz.org