This patient has melanonychia, and she requires a nail matrix biopsy. Melanonychia refers to a brown or black discoloration of the nail plate. It is a common normal variant in persons with darker skin types, but it may also occur as a result of systemic disease, medication, infection, or an underlying melanocytic lesion. Typically, multiple nails are affected. A single nail with longitudinal melanonychia is suggestive of an underlying melanocytic lesion. In this setting, it is important to rule out subungual melanoma. Clinical signs of subungual melanoma include Hutchinson sign (pigmentation expanding onto the proximal nail fold) and a wider diameter of the longitudinal pigment stripe proximally as compared with distally (indicative of an expanding underlying pigmented lesion in the nail matrix). Subungual melanoma may also present as diffuse nail discoloration and is often initially misdiagnosed as onychomycosis or subungual hematoma. Although rare in patients with fair skin, subungual acral lentiginous melanoma is the most common type of melanoma in Asian and black patients.

Onychomycosis is the result of dermatophytes invading the nail plate and causing thickening and discoloration. The diagnosis of onychomycosis can be confirmed by potassium hydroxide (KOH) examination of the nail, a fungal culture, or by histologic examination of the nail clippings. The finding of a pigmented linear streak is not consistent with the diagnosis of onychomycosis, and these studies are not indicated.

A culture of the nail clipping is not appropriate because the nail plate changes are not consistent with an infection such as onychomycosis.

Histopathologic examination of the nail clipping is not sufficient to determine the presence or absence of a melanoma since other disorders can cause pigment deposition in the nail plate. Observation is not appropriate in this situation, since a delay in diagnosis can have potentially serious consequences.