Spirometry — Mannitol Indirect Bronchoprovocation

A 22-year-old male athlete presents with a 6-month history of exertional dyspnea and cough during competitive running. Spirometry at rest shows normal FEV1 (92% predicted). Methacholine challenge is not available at your centre. A mannitol bronchial challenge test is performed using escalating dry-powder doses. The patient's FEV1 remains unchanged at 5 mg mannitol (point B), drops 8% at cumulative dose 155 mg (point C), and falls 16% from baseline at cumulative dose 315 mg (point **D**), at which point the test is stopped and salbutamol is administered. The finding at point **D** (PD15 reached at 315 mg cumulative dose with ≥15% FEV1 fall) in this athlete is most consistent with which of the following?