black urine

a 2-year-old boy was born at term of healthy, non-consanguineous iranian parents. his mother attended in the clinic with the history of sometimes discoloration of diapers after passing urine. she noticed that first at the age of one month with intensified in recent months. his physical examination and growth parameters were normal. his mother denied taking any medication (sorbitol, nitrofurantoin, metronidazole, methocarbamol, sena and methyldopa (5)). qualitative urine examination showed dark black discoloration. by this history, alkaptonuria was the most clinical suspicious. a 24-hour-urine sample was collected and sent for quantitative measurements. the urine sample was highly positive for homogentisic acid and negative for porphyrin metabolites.