Fingernail psoriasis. Close-up of the pitted fingernails of a patient with psoriasis. Psoriasis is a common skin disease involving a ten-fold increase in the speed of epidermal cell proliferation. Here, psoriasis has caused pitting of the nails where the nail keratin is less well knit because of this accelerated growth. Growth may also be distorted with the nails being thick and friable. Nails are also characteristically loosened from the nailbeds. The cause of psoriasis is unknown but certain conditions including infection, some drugs and stress are known to trigger attacks.