alkapton

from The Century Dictionary.

• noun A term originally introduced to designate a certain urinary constituent which is met with on rare occasions, and which causes the urine to turn reddish brown or black on standing or upon the addition of an alkali.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun homogentisic acid

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an acid formed as an intermediate product of the metabolism of tyrosine and phenylalanine