A 28-year-old woman presents with a 3-month history of intermittent nocturnal cough and dyspnea on exertion. She denies wheeze. Baseline spirometry shows FEV1 88% predicted with FEV1/FVC ratio 0.78. A methacholine challenge test is performed. The result marked **C** in the diagram — PC20 <8 mg/mL — is obtained. Which of the following best explains the diagnostic significance of this finding in the context of her clinical presentation?