A mother brings her 13-year-old daughter to the office. The girl has had intermittent lower abdominal pain, constipation, and difficulty voiding for 3 months. She says that she is not sexually active. She looks well. She has reached age-specific developmental milestones, and her vital signs are within normal range. On abdominal examination, she is found to have a palpable suprapubic mass that persists after voiding. The girl says that her older sister started having menstrual periods at this age. The patient is surprised that hers have not started. Which one of the following is the best next step?
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