Phenylketonuria and Alkaptonuria both are inborn errors (congenital) of metabolism. Phenylketonuria is due to accumulation of phenylpyruvic acid and causes mental disorders. Alkaptonuria is the lack of ability to breakdown homogentisic acid into acetoacetic acid. Due to accumulation of homogentisic acid, the urine of patients suffering from this disease turns black as soon as it comes in contact with air.