A 72-year-old man is admitted with pneumonia after three days of fever, cough and increasing breathlessness. He is alert, but his respiratory rate is 34/min and oxygen saturation is 88% while breathing room air; he requires 6 L/min of oxygen to maintain a saturation of 94%. Chest radiography shows bilateral air-space opacities, and his blood pressure is 118/70 mmHg with a normal lactate concentration. Which one of the following is the most appropriate initial antibiotic regimen?