A 64-year-old woman is admitted to the hospital after the acute onset of left-sided weakness. She has had a large right cerebral stroke and is confined to bed. On the fifth hospital day, her oxygen saturation is noted to be reduced to 90% on room air. She feels fine, the neurologic weakness is unchanged, blood pressure is 130/90 mm Hg, and pulse 80/min. Examination of the chest reveals decreased fremitus, dullness to percussion, and absent breath sounds in the left lower lung. There is also a tracheal shift towards the left side.For the above patient with abnormal pulmonary physical findings, select the most likely diagnosis.