A 28-year-old female nurse presents with recurrent episodes of sudden inspiratory stridor and throat tightness triggered by strong odors and workplace stress, resolving with distraction or pursed-lip breathing. She has been intubated twice for presumed asthma, but baseline spirometry is normal and methacholine challenge is negative. During a symptomatic episode, the flow-volume loop demonstrates the pattern shown. The structure marked **A** (flattened inspiratory loop) represents which of the following pathophysiologic mechanisms?