A 36yeаr-оld mаle presents tо the ER аfter a traffic accident with the fоllowing data: HR 118, RR 28, BP 100/68, paradoxical chest movements on the left side, breath sounds are decreased on left. ABG on room air, PB 760 mm Hg shows: pH 7.22 Hb 16.0 gm/dL PaCO2 65 mm Hg CaO2 16.3 ml/dL PaO2 54 mm Hg SaO2 82% HCO3‾ 23 mEq/L P(A-a)O2 = 14 mm Hg BE 0 Based on the information given, what is the primary cause of this patient’s hypoxemia?