A 25-year-old woman presents with a 6-week history of persistent dry cough, particularly worse at night and after exercise. She also reports some chest tightness. She has a history of hay fever. There are no signs of infection. Spirometry shows an FEV1/FVC ratio of 0.65, which improves by 15% (and 250ml in FEV1) after administration of a bronchodilator. What is the most likely diagnosis?