• Extracellular matrix
• histopathology
• inherited pathology
• metabolism

Alkaptonuria (AKU) is a rare autosomal recessive condition with a prevalence of 1 in 250 000–1 000 0001 caused by the deficiency of homogentisate 1,2 dioxygenase, the enzyme responsible for breaking down homogentisic acid (HGA).2 Without this enzyme, levels of HGA within the tissues increase, even with highly efficient renal clearance of HGA. Urinary excretion of HGA is the only manifestation of AKU in children, and symptoms appear in adults due to joint damage. HGA has a high affinity for collagenous tissues, primarily the articular cartilages where it deposits as a pigmented polymer. Deposition also occurs at non-collagenous sites, suggesting that factors other than the presence of HGA in the tissues may favour or promote ochronosis.2 3 Ochronotic pigment does not stain with Perl's reaction and can be highlighted in histological sections by Schmorl's stain.4

We report the case of a 51-year-old woman with shoulder joint arthritis who had been diagnosed previously with AKU elsewhere at the age of 50 years; the diagnosis was confirmed by urinary measurement of HGA. She attended the Liverpool AKU clinical evaluation unit where routine assessment by whole-body MRI revealed a 9 cm diameter, partly calcified mass in the anterior mediastinum (figure 1). In 1995, she was diagnosed with hyperthyroidism and had …