A 31-year-old man presents to the clinic for evaluation of worsening shortness of breath on exertion. He has severe kyphoscoliosis due to cerebral palsy. He reports no other symptoms of chest discomfort, fever, chills, cough, or sputum production. On physical examination, he is not in distress; he has a severe scoliosis to the left; and decreased air entry to that side. His right lung is clear, the JVP is at 3 cm, and heart sounds are normal. Pulmonary function tests are performed. Which of the following is the most likely abnormality to be seen on the pulmonary function tests?