Median Canaliform Dystrophy
MCD is a condition wherein the nail matrix (NOT nail bed, NOT nail plate, NOT proximal nail fold) is damaged, Often through trauma, but it can also be idiopathic or can be related to genetics. Think of the nail as a rainbow. It is formed as ROYGBIV and arcs across the sky, but always the orientation remains the same: ROYGBIV. So, when you have a MCD as above, the defect is at G (in other words, pretty much in the middle of the nail matrix). This patient also has a paronychia, but that was a red herring.
The weird thing about these defects is that they often wax and wane, and may disappear, only to recur years later.