Gout — Tophi on Helix of Ear and First MTP

A 62-year-old man with a 15-year history of poorly controlled gout presents with multiple firm nodules on his ears, fingers, and elbows. On examination of the left ear helix, a nodule is noted to have ulcerated and is discharging a chalky white material. Microscopic examination of the material marked **D** in the diagram shows needle-shaped crystals that appear yellow when the compensator is parallel to the crystal axis under polarized light microscopy. Which of the following best describes the chemical composition and pathophysiology of this discharged material?