Alkaptonuria

Alkaptonuria is characterised by the accumulation of homogentisic acid (HGA) and its oxidised product benzoquinone acetic acid (BQA), leading to a darkening of the urine when it is left exposed to air, grey-blue colouration of the eye sclerae and the ear helix (ochronosis), and a disabling joint disease involving both the axial and peripheral joints (ochronotic arthropathy).