Q. A 35-year-old man comes to the clinic with an unusual complaint. He has noticed that his urine turns dark, almost black, when left standing for a prolonged time. He also mentions that he has experienced intermittent joint pain, especially in his knees. On physical examination, you notice a bluish-black discoloration of his ears and sclera. His medical history is otherwise unremarkable, and he has not taken any medication recently. Which enzyme deficiency is most likely responsible for these findings?