Clinical Presentation of Alkaptonuria:Normal life till 3rd or 4th decade Urine darkens on standing is the only manifestation in children. In adults, ochronosis, i.e. Alkaptone body deposition in intervertebral disk, cartilage of nose, pinna, etc. leading to pigmentation. Arthritis. No mental retardation.Laboratory Diagnosis:Alkalinization increases the darkening of urine. Benedict's test positive in urine because homogentisic acid is a reducing agent. Ferric chloride test positive. Silver nitrate test positive.Note: Guthrie bacterial inhibition test is a screening test for phenylketonuria. Reference- Harper’s illustrated biochemistry. 30th edition page -304