Bacteriology USMLE Step 1 Practice Question

A 28-year-old woman with no significant past medical history presents to the emergency department with a 3-day history of high fever, productive cough with purulent sputum, and dyspnea. She reports recent upper respiratory symptoms that progressed to lower respiratory involvement. Vital signs: temperature 39.2°C, heart rate 112/min, respiratory rate 24/min, blood pressure 118/76 mmHg, and SpO2 94% on room air. Chest X-ray shows patchy alveolar infiltrates in the left lower lobe consistent with community-acquired pneumonia. Sputum Gram stain reveals numerous neutrophils and gram-negative rods. Culture of sputum on MacConkey agar grows pink, mucoid colonies. The organism ferments lactose, oxidizes glucose, and demonstrates motility with peritrichous flagella. Blood cultures remain sterile after 48 hours. Which of the following is the most appropriate empiric antibiotic therapy for this patient?