Ashort, nonsmoking middle-aged man presents to the emergency room with left-sided chest pain and a cough. he says that the pain started abruptly, just after lunch, and that breathing and coughing make it worse. he denies recent injury. he is breathing shallowly and rapidly and expresses fear that he may be having a "heart attack." breath sounds are normal, and he is not cyanotic. which condition is most likely causing his symptoms?