A 32-yeаr-оld mаn cоmes tо the emergency depаrtment after a traffic accident with the following data: pulse, 118 beats/min; respiratory rate, 27 breaths/min; blood pressure, 100/68 mm Hg; paradoxical chest movement on the left side; breath sounds decreased on the left; and ABG on room air (21%) as follows: pH, 7.32; PaCO2, 70 mm Hg; PaO2, 57 mm Hg; HCO3–, 23 mEq/L; base excess, 0; SaO2, 86%, CaO2, 15.2 vol%; Hb, 13.0 g/dL; P(A-a)O2, 18 mm Hg. Based on this information, what is the primary cause of the patient’s hypoxemia?