A 28-year-old woman presents to the emergency department with a 5-day history of a painful, erythematous, fluctuant lesion on her right forearm. She reports that it started as a small pustule after a minor cut while gardening. On examination, the lesion is 4 cm in diameter, warm, and tender, with surrounding cellulitis. Gram stain of the aspirate shows gram-positive cocci in clusters. Blood culture is negative. What is the most appropriate next step in management?