A 2-year-old girl is evaluated for cough, runny nose, watery eyes, and fevers for a week. She also developed a diffuse rash, which first appeared behind her ears and then spread to the chest, arms, and legs. Her mother notes that the patient also had white spots on the buccal mucosa several days before the rash developed. She has no sore throat or difficulty swallowing. The patient has no pertinent past medical or family history. On physical examination, the vital signs include a temperature of 98.9 °Fahrenheit, pulse rate of 98/min, and blood pressure of 101/81 mm Hg. Skin evaluation is notable for a diffuse, erythematous, and maculopapular blanching rash. Which of the following is the most appropriate next test?
1) Rubella IgM
2) Group A streptococcus rapid antigen detection test
3) Heterophile antibody testing
4) Measles IgM