A 23-year-old man notices a gradual but progressive increase in breathing difficulty. He has a long history of back pain with prolonged morning stiffness. He has also had an episode of iritis in the past. On examination, there is reduced range of motion in the lumbar spine with forward flexion and pain on palpation of the sacroiliac joint and surrounding soft tissue. X-rays of the pelvis show erosions and sclerosis of the sacroiliac joint. Which of the following is the most likely pulmonary complication of this condition?