Which condition is characterized by the overgrowth of the keratin layer of the nail?

Nail psoriasis is characterized by the overgrowth of the keratin layer of the nail. This condition is a manifestation of psoriasis, which is an autoimmune disorder leading to inflammation and rapid skin cell turnover. In the case of the nails, this results in thickened, discolored nails that may have pitting, ridges, or separation from the nail bed. The overproduction of keratin, driven by the underlying inflammatory process, causes the nail plate to grow abnormally thick and can lead to visible changes in the nail’s structure and appearance.

The other conditions listed do not primarily involve the overgrowth of the keratin layer. For instance, onychomycosis refers to a fungal infection of the nails, which leads to discoloration and thickening but does not primarily stem from keratin overproduction. Onychophagy is the habit of nail biting, resulting in nail damage but not an increase in keratin deposition. Pyogenic granuloma is a type of growth that presents as a red, swollen area and can occur in various locations, including the nails, but does not pertain to keratin overgrowth specifically. Therefore, nail psoriasis accurately fits the description of overgrowth of the keratin layer of the nail.