Alkaptonuria is an autosomal recessive inborn error of metabolism caused by the accumulation of homogentisic acid due to a deficiency of an enzyme called HGO (homogentisate 1.2-dioxygenase).

It is characterized by: 
  • Urine that turns dark on standing 
  • Dark pigmentation of cartilage and connective tissue (ochronosis) 
    • In MRCP classically look for pictures with a dark patch on the sclera or pinna 
  • Arthritis 

It is also associated with: 
  • Renal stones 
  • Aortic dilation
  • Deterioration of cardiac valves 

Spinal xrays show intervertebral calcification 

  • No definitive treatment at present. 
  • No clear benefit from low-protein diet or vitamin C supplements 
  • Possibly nitisinone in the future 

Small print gems: the pigmentation is called ochronosis because when viewed microscopically it appears ochre, despite appearing grey-black to the naked eye.  It is sometimes called Garrod's disease after the doctor who discovered it.

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Secret collector of interesting anonymised ECGs. Fan of the Bath Photomarathon. Lover of cream teas. [Sarah Hudson] (Your Picture)