A 32-year-old woman presents with a painful, fluctuant swelling on her left forearm that appeared 5 days ago. She has fever (38.5°C) and mild leukocytosis. On examination, the lesion is warm, erythematous, and tender. Gram stain of aspirated pus shows gram-positive cocci in clusters. Which investigation is most appropriate to confirm the diagnosis and guide antibiotic therapy?