Dark Streak Traversing the Thumb Nail

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A 53-year-old Hispanic woman requests treatment for idiopathic guttate hypomelanosis, or white spots, on her legs. During the physical examination, a dark streak is observed traversing the patient’s right thumb nail. The patient works as a laborer but denies any trauma to the site. She d

Longitudinal melanonychia, also known as melanonychia striata, is characterized by a brown or black discoloration traversing the nail lengthwise. The prevalence of longitudinal melanonychia varies by race; the condition is most common in black individuals followed by Hispanic and Asian individuals.1,2 The condition results from deposits of melanin in the nail plate that is produced by melanocytes located in the suprabasal layer of the nail matrix.1,3

Differential diagnosis of longitudinal melanonychia includes hemorrhage secondary to trauma and pigmentation induced by drugs or other malignancies.4,5 Although the majority of longitudinal melanonychia cases are benign, subungual melanomas represent a significant subset.6 Dermoscopic examination of subungual melanoma may reveal irregular pigmentation and early Hutchinson sign (pigmentation extending into the nail folds); however, definitive diagnosis requires histopathologic examination.7,8 Melanoma of the nail, although most common in adults aged 50 to 70 years, can also occur in children.9

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