A 63-year-old man with small cell carcinoma of the left mainstem bronchus begins chemotherapy. During the treatment period, he becomes febrile and develops a productive cough. The temperature is 38.7degC (103degF), respirations are 32 per minute and blood pressure is 125/85 mm Hg. A CBC shows leukocytosis (WBC = 18,500/mL). The patient's cough worsens and he begins expectorating large amounts of foul smelling sputum. A chest X-ray shows a distinct cavity with an air/fluid level distal to the tumor area. Which of the following is the most likely diagnosis?