A 70-year-old man with known chronic obstructive pulmonary disease presents with 3 days of increased dyspnoea, sputum volume and purulence. He is afebrile, saturating 93% on room air and there is no radiographic evidence of pneumonia. You diagnose a moderate COPD exacerbation. In addition to optimising bronchodilators and a short course of oral prednisolone, which oral antibiotic is the most appropriate first-line choice?