A 3-year-old American girl presents to the emergency department with fever, rash, conjunctivitis and swollen hands and feet. She has been feverish for 5 days, and her fever is now 39.0^o C (102.2^o F). She has been coughing but denies any difficulty breathing. Her pulse is 140/min and her blood pressure is 95/55 mm Hg. She has a non-pruritic maculopapular rash on her trunk, face, hands and feet. Her lymph nodes are tender but non-enlarged. Her laboratory results show a white blood cell count of 15,000/mm3 with 80% neutrophils and C-reactive protein of 10 mg/dL. Her echocardiogram shows normal left ventricular function with no evidence of an aneurysm. An ECG is normal. Which of the following is the most appropriate next step in her management?