A 5 year-old girl is referred for evaluation of bed-wetting. The mother states that her daughter has never fully gained control of her bladder. It took her an extended period of time to grow out of diapers and the girl has finally stopped having "accidents" during the day. However, she continues to wet her bed at night. The patient has developed normally in all other aspects and will be staing kindergarden in three months. Both her mother and father are extremely frustrated and have been losing sleep, as the girl wakes up at night 4 to 5 times per week. Physical examination is unremarkable. Which of the following will most likely be the result of fuher investigation?