A 12-year-old girl with an unremarkable history was evaluated for a nodule of the lateral nail fold on the right hallux that had been present for 3 months (Panel A). The lesion, which was initially believed to be a wart, was treated with a topical salicylic acid preparation, with no improvement. The nodule measured 10 mm in diameter, and the distal nail plate showed onycholysis and had been destroyed. A radiograph revealed a calcifying projection on the dorsolateral part of the distal phalanx, continuous with the underlying bone (Panel B), the features of which were consistent with a subungual exostosis. . . .