A 64 year old man presents to the physician's office complaining of fevers for the past 2 days. Over the past 24 hours, he has developed a productive cough. He also repos that he has frequent chills, and has been waking for the past 2 nights with drenching sweats. His past medical history is remarkable only for mild exeional angina. On physical examination, he does not appear chronically ill but appears moderately dyspneic. His temperature is 38.6 C (101.4 F), blood pressure is 136/94 mm Hg, and respirations are 26/min. There is no jugular venous distention. The lungs have coarse rhonchi at the right lung base with increased fremitus in the same area. He has a regular hea rhythm, with a 1/6 systolic murmur at the left sternal border. The remainder of the physical examination is unremarkable. Which of the following is the most appropriate next step in diagnosis?