Alkaptonuria Ochronosis Blue-Grey Ear

A 52-year-old man with a lifelong history of dark urine presents to the rheumatology clinic with progressive low back pain, large joint involvement, and recent hip and knee pain requiring arthroplasty evaluation. On examination, the structure marked **A** (slate-grey to blue-black discoloration of the auricular cartilage) is noted bilaterally with preserved normal earlobe skin color. His urine darkens on standing, and laboratory studies confirm elevated urinary homogentisic acid (>1 gram per 24 hours). Genetic testing reveals biallelic HGD gene mutations. Which of the following best explains the cartilage-specific pigmentation seen in the structure marked **A**?