USMLE Step 1 Practice Test 6

A 63-year-old man is brought to the emergency department because of 3 days of fever, nonproductive cough, watery diarrhea, and confusion. He smokes 1 pack of cigarettes daily. He recently returned from a conference at a hotel where several other guests became ill with pneumonia. Temperature is 39.8°C (103.6°F), pulse is 84/min, respirations are 26/min, and blood pressure is 118/70 mm Hg. Crackles are heard over the right lower lung field. Serum sodium concentration is 126 mEq/L, and serum AST and ALT activities are mildly increased. Gram stain of sputum shows many neutrophils but few organisms. Which of the following is the most likely causal organism?