A 32-year-old woman with a 6-month history of episodic nocturnal cough and dyspnea presents for evaluation of suspected asthma. Spirometry shows normal baseline FEV1 (88% predicted) with no bronchodilator reversibility (FEV1 increase 8%). A methacholine challenge test is performed. The patient develops a 20% drop in FEV1 at a methacholine concentration of 2.5 mg/mL, as marked by **C** in the diagram. Which of the following best describes the diagnostic significance of this finding?