A 35-year-old male presents with a 3-day history of fever, productive cough, and pleuritic chest pain. Chest X-ray shows a left-sided pneumonia with a small pleural effusion. Blood cultures are pending. Sputum Gram stain shows numerous gram-positive cocci in clusters. Which investigation is most appropriate to confirm the diagnosis of Staphylococcus aureus pneumonia and guide antibiotic therapy?