Alkaptonuria

A 50-year-old man comes to the clinic for evaluation of chronic back pain. He works as a librarian and notes that the pain is localized to his lower back and worsens after a long day of standing. The pain has not improved despite treatment with ibuprofen. Past medical history is significant for osteoarthritis of his right hip, for which he underwent a total hip replacement 2 years ago. Family history is noncontributory. Temperature is 37.0°C (98.6°F), pulse is 86/min, respirations are 16/min, and blood pressure is 125/85 mmHg. Ophthalmic examination reveals a bluish pigment deposition in the sclera. Dermatological examination reveals hyperpigmentation in the axillary and inguinal regions. Range of motion at the spine is limited. An x-ray of the erect spine is shown below:


Osmosis High-Yield Notes

Laboratory evaluation reveals elevated levels of homogentisic acid. The production of which of the following end products is likely to be inhibited, considering the most likely diagnosis?