A 4-year-old boy presents with fever for 6 days, bilateral non-purulent conjunctivitis, polymorphous truncal rash, and edema of hands and feet. On examination, the structure marked **A** in the diagram shows a bright red surface with prominent papillae, and he has red cracked lips. Blood cultures are sterile, and throat culture is negative for Group A Streptococcus. Which of the following is the most appropriate next step in management?