Alkaptonuria is one of the genetic diseases that occur with there is Homogentisic acid present in your urine. It is a rare condition that arises due to the absence of enzyme Homogentisic Acid Oxidase. The primary symptom of this condition is the black color of the urine when it is exposed to air. However, sometime you might not check it, and thus the other symptoms include the black sweat, dark spot in the sclera of the eyes, discoloration of the skin, hip and joint pain. If you have such conditions and also suffering from the heart issues where the valves of your heart have become thick, then you must go for the homogentisic acid in urine.