A 7-year-old child is taken to the emergency depament because he is feeling sho of breath. The episode began about an hour previously while the child was playing spos, when he abruptly developed paroxysms of wheezing and coughing. As the physician enters the room, he notes that the child is sitting leaning forward and is using his accessory respiratory muscles. Physical examination demonstrates tachypnea and tachycardia. On auscultation, a prolonged expiratory phase with relatively high-pitched wheezes through much of the respiratory cycle are heard. No fine crackles are heard. Which of the following medications will have the fastest onset if the 7-year-old child has an acute attack of his condition?