The so-called "psoriatic crumb nail" is a severe combination damage of the nail organ. A severe and extensive psoriatic affection of the proximal nail matrix (a punctual disturbance would lead to spotted nails) results in a parakeratotic nail plate with exfoliation of horny platelets layered like roof tiles.
If the disorder persists for a longer period of time, the hard, dorsal part of the nail plate refoliates completely. This leads to keratinization disorder and exfoliation of the distal nail plate as well, whereby the connective tissue structures of the nail bed are exposed either selectively or over a large area. The result is potential focal oozing bleeding.