A mother arrives to your office in a panic. She had just returned home from the delivery of her third child, who was born by cesarean section because he was large for gestation age. Upon changing his first diaper at home she noticed gross blood in his diaper. On examination, you find a left-sided abdominal mass that was not present in the normal newborn nursery. Which of the following is the most appropriate next step in the diagnosis and management of this child?