Gross — Glomus Tumor Subungual Painful Pink Lesion

A 42-year-old woman presents with a 3-year history of severe, paroxysmal throbbing pain in her right index fingertip, markedly worsened by cold exposure. On examination, a small bluish-pink nodule is visible through the nail plate at the distal fingertip (marked **A** in the diagram). Point pressure over the lesion with a pin reproduces the severe pain. Inflation of a blood pressure cuff above systolic pressure on the proximal arm immediately abolishes the pain. Which of the following best describes the pathological origin of the structure marked **A**?