A 32-Year-Old Man Comes to the Emergency Department After a Traffic

A 32-year-old man comes to the emergency department 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?

A) Overall hypoventilation
B) mismatch
C) Diffusion defect
D) Shunt