Since birth, a 2-year-old female toddler has complained of blackish discoloration on wet nappy drying. She was the sole kid of a nonconsanguineous couple, and she was doing well. Her developmental milestones were all within normal limits. There were no abnormalities discovered during the clinical assessment. The urine sample was tested for silver nitrate and Benedict's test, which both came back positive. a) What would your first diagnosis be?
b) What tests would you recommend in order to confirm your diagnosis?