What causes brown or dark lines under the nail?

The correct answer is Melonychia. This condition specifically refers to the presence of brown or dark pigmentation in the nail plate and can occur for several reasons, including excess melanin production in the nail matrix. The pigmentation can reflect various underlying issues, including benign causes like trauma or more serious conditions such as melanoma. Understanding that melonychia is primarily characterized by the discoloration of the nail helps in identifying potential health concerns related to the nails and carrying out appropriate consultations or treatments.

In contrast, pterygium is a condition involving the overgrowth of cuticle tissue onto the nail plate, which does not lead to discoloration. Aging affects the nails in various ways, often causing them to become thinner or more brittle, but it is not specifically responsible for dark lines. Nail fungus typically results in a thickened, discolored nail that can turn yellow or green, rather than causing brown or dark lines specifically. Thus, it is melonychia that accurately describes the phenomenon in question.