A 7-year-old boy with a 2-year history of episodic wheeze triggered by viral upper respiratory infections, exercise, and cold air presents to the pediatric clinic. Spirometry shows an obstructive pattern with FEV1/FVC ratio of 0.78. After administration of a short-acting beta-2 agonist via spacer, repeat spirometry shows FEV1 increase of 15% from baseline. The condition marked **A** in the diagram demonstrates this characteristic finding. Which of the following best describes the pathophysiological basis of the bronchodilator reversibility observed in this child?
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