Review Question - QID 210403

A 55-year-old African American man presents to his pulmonologist for dyspnea. He underwent a lung transplant 3 years prior to presentation, and over the past 3 months, he has noticed worsening dyspnea and a dry cough that he did not have after the transplant. He was initially diagnosed with rapidly progressive and treatment-resistant idiopathic pulmonary fibrosis 7 years ago and experienced a rapid decline in function since the diagnosis. He was previously very active in his neighborhood's running club and worked as an investment banker. He was placed on the transplant list 2 years after his diagnosis and underwent a lung transplant 3 years ago. He has been on tacrolimus, mycophenolate, and prednisone since the transplant and has tolerated the drugs well. His temperature is 98.6°F (37°C), blood pressure is 125/75 mmHg, pulse is 98/min, and respirations are 22/min. Results of pulmonary function tests are shown below:

FEV1: 21% predicted
FVC: 80% predicted
FEV1/FVC: 30%
DLCO: Normal

A chest radiograph in this patient is normal. A computerized tomography (CT) scan of the patient's lungs taken at end-expiration is shown in Figure A. Which of the following parts of the tracheobronchial tree is most likely affected in this patient?