Habit tic deformity
The habit tic deformity is a self-induced condition that is caused by damage to the external part of the nail matrix, usually by pressing or scraping on the lunula of the nail with another fingernail. Since the matrix is softer than the fully-formed nail, it is more easily damaged than the distal nail. The way to tell it is a habit tic deformity is to see if the dystrophic changes extend all the way across the nail, as they would with chronic paronychia, or if they are longitudinal and well defined, like it would be with a median canaliform dystrophy. Also, with chronic paronychia, one would see interruption or complete disruption of the proximal nail fold.
Median canaliform dystrophy
Use this as a moment to reflect on the way the nail is made. The lunula, which is largely obscured by the proximal nail fold, is the matrix. The most proximal of the lunula/matrix ends up being the outer part of the nail plate, the distal part of the lunula/matrix is the deep part of the nail plate. So, since this dude was primarily picking at the distal part, the changes of the dystrophy actually go all the way through the nail. To compare it to the rainbow below, the red part is originated at the proximal matrix, the violet part is formed at the distal part of the matrix.
We are coming down to the final days of March Madness, and we have a two-way tie for the lead, with two others one point behind. Who will win the Gewurztraminer? Stay tuned...