MRCP2-4300

A 35-year-old construction worker presents to the Pulmonary Clinic for evaluation. He has been previously diagnosed with asthma by his primary care physician and is currently being treated with high-dose fluticasone/salmeterol and salbutamol, but his symptoms have not improved. Upon examination, his blood pressure is 120/70 mmHg, with a regular pulse of 75/min. Coarse crackles and scattered wheezing are heard during chest auscultation. Respiratory function tests reveal an obstructive pattern. A thoracic HRCT scan shows centrally dilated thickened airways with signet rings. Which test would be the most helpful in confirming the suspected diagnosis?