A 36-year-old woman with a history of obstructive sleep apnea is admitted with acute shoness of breath, cough with greenish sputum, and fever. On physical examination, she is febrile and has decreased breath sounds generally and diffuse bilateral rhonchi. ABGs: pH 7.32; PCO2 47 mm Hg; PO2 65 mm Hg with O2 saturation 87%.The patient improves initially but has a respiratory arrest as she is being moved to the ICU, requiring emergency endotracheal intubation. While a CXR is ordered, it is noted that she has absent breath sounds on the left side. CXR shows