A 29-year-old man presents to the clinic because he is concerned about blue fingers. He reports no other symptoms and is otherwise healthy. On physical examination, his vital signs are normal except for an oxygen saturation of 87% on room air. His lung and cardiac exams are normal but there is bluish discoloration of his fingers consistent with cyanosis. A blood gas reveals hypoxemia and hypercapnia, while the CBC suggests polycythemia. With voluntary hyperventilation he is able to restore his blood gases to normal. Which of the following is the most likely location for the abnormalities seen on his blood gases?