A 52-year-old man with a 15-year history of primary osteoarthritis of the right knee presents with progressive pain and functional limitation. Plain radiographs show joint space narrowing, osteophyte formation, and subchondral sclerosis. He has failed conservative management including NSAIDs and physiotherapy. During preoperative assessment for total knee arthroplasty, his serum uric acid is found to be 8.2 mg/dL (normal <7 mg/dL). Which of the following best explains the elevated serum uric acid in this patient?