A man complains of unbearable cough with putrid-smelling chocolate-colored purulent sputum. The discharge of sputum reaches 600 mL. per 24 hours. The patient's history states that the disease onset was acute, with irregular fever and the body temperature reaching 39°C. X-ray shows a shadowed area with a cavity in its center that has irregular contours and a fluid level. What is the most likely diagnosis in this case?