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A previously healthy 15-year-old boy presented with erythema migrans, a characteristic skin rash that often appears as the earliest manifestation of Lyme disease (LD) (online supplemental figure S1). He was prescribed doxycycline 2 mg/kg orally two times a day for 14 days. Subsequently, he noticed painless dark stains on all his fingernails (figure 1 and online supplemental figure S2).SP110.1136/archdischild-2025-330043.supp1Supplementary data