Which nail disorder causes the nail plate to darken?

The condition that causes the nail plate to darken is melanonychia. This term specifically refers to a brown or black discoloration of the nail, which occurs due to an increased amount of melanin in the nail matrix. Melanonychia can be caused by various factors, including certain medical conditions, trauma to the nail, or even pigmentation changes related to benign or malignant processes.

In contrast, while nail psoriasis can lead to changes in the appearance of the nails such as pitting or discoloration, it does not specifically cause the entire nail plate to darken in the way that melanonychia does. Ridges can occur in nails due to aging or other non-specific reasons, but they do not cause darkening. Fungal infections can also cause discoloration of nails, but they typically lead to a yellow or white appearance rather than the characteristic darkening associated with melanonychia.