Spirometry — Variable Intrathoracic Obstruction

A 52-year-old man with a 10-year history of COPD presents with progressive dyspnea on exertion and expiratory stridor over the past 3 months. His symptoms have NOT improved with high-dose inhaled bronchodilators and corticosteroids. Spirometry shows a flow-volume loop with a characteristic flattened expiratory limb while the inspiratory limb remains preserved. The pattern marked **C** in the diagram represents the underlying pathophysiology. Which of the following is the MOST LIKELY diagnosis?

A 58-year-old man with a 10-year history of "asthma" presents with progressive dyspnea, chronic cough, and monophonic wheeze that has worsened despite high-dose inhaled corticosteroids and long-acting bronchodilators. His flow-volume loop shows a flattened expiratory limb with a normal inspiratory loop. The pattern marked **D** in the diagram suggests an intrathoracic airway lesion. Which of the following is the MOST appropriate next diagnostic step to identify the underlying cause?