A 28-year-old Indian man presents with progressive darkening of urine over the past 5 years, which turns black upon standing or exposure to air. He reports chronic joint pain affecting the spine and large joints (knees, hips, shoulders). Physical examination reveals a dark pigmentation of the sclera and ears (ochronosis). Serum uric acid is elevated at 8.2 mg/dL. Urine testing shows a positive ferric chloride test (turns dark purple-brown). What is the most likely diagnosis?