Case: A 62-year-old man presented with a 10-year history of isolated melanonychia striata of his dominant thumb. Surgical biopsy ruled out subungual melanoma but revealed foreign plant material causing chronic infectious melanonychia from multiple pathogens, including Pseudomonas aeruginosa, Escherichia coli, and Candida spp. After removal of the nail plate and thorough debridement, the melanonychial streak resolved completely at 12 months of follow-up.
Conclusion: Bacterial infection is a rarely reported cause of melanonychia, and in addition to surgical pathologic specimens, intraoperative fungal and bacterial cultures should always be obtained for accurate diagnosis of melanonychia striata.
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