A man in his 70s presented to the clinic with a split in the right fifth fingernail for the past 10 years. He reported paroxysmal, intermittent pain that was exacerbated by cold weather. He fractured the digit more than 30 years earlier. Physical examination revealed distal nail plate splitting with accompanying longitudinal erythronychia (Figure , A) and tenderness on palpation of the nail unit. Magnetic resonance imaging revealed a nail matrix lesion (Figure , B), which was excised using a transungual approach with partial nail avulsion. The specimen was analyzed by histopathology (Figure, C and D). Histopathologic image D Histopathologic image C Magnetic resonance image B Physical examination A Figure. A, Right fifth fingernail with distal nail plate splitting and longitudinal erythronychia. B, Magnetic resonance image shows localization of the tumor to the nail matrix. C, Hematoxylin-eosin stain (original magnification ×4). D, Hematoxylin-eosin stain (original magnification ×20). WHAT IS YOUR DIAGNOSIS? A. Amelanotic melanoma B. Onychopapilloma C. Glomus tumor D. Squamous cell carcinoma in situ Clinical Review & Education