Q. A 9-year-old girl is brought to the pediatrician by her mother due to recurrent episodes of coughing, wheezing, and shortness of breath, particularly after exercise and during the night. Her mother reports that the girl's father has a history of allergic rhinitis. The girl's vital signs are as follows: temperature 36.7°C (98.1°F), heart rate 88 beats/min, respiratory rate 24 breaths/min, and blood pressure 100/60 mm Hg. On physical examination, the girl has expiratory wheezing and mild use of accessory muscles for breathing. The pediatrician suspects asthma and orders a test to confirm the diagnosis. Which of the following tests would be most appropriate for confirming the diagnosis of asthma in this patient?