Alkaptonuria, or "black urine disease", is a very rare inherited disorder that prevents the body fully breaking down two protein building blocks called tyrosine and phenylalanine.
It results in a build-up of a chemical called homogentisic acid in the body.
This can turn urine and parts of the body a dark colour and lead to a range of problems over time.
Amino acids are usually broken down in a series of chemical reactions. But in alkaptonuria, a substance produced along the way, homogentisic acid, cannot be broken down any further.
This is because the enzyme that normally breaks it down does not work properly.
One of the earliest signs of the condition is dark-stained nappies, as homogentisic acid causes urine to turn black when exposed to air for a few hours.
If this sign is missed or overlooked, the disorder may go unnoticed until adulthood, as there are usually no other noticeable symptoms until the person reaches their late 20s to early 30s.