A 4-year-old female is brought by her father to the emergency department after experiencing 3 days of worsening sore throat and a fever as high as 103.1° F. She has constant generalized abdominal pain, intermittent mild headache, and decreased oral intake, and her throat hurts more than her head or abdomen. She has no cough, trouble breathing, or runny nose, and she has no vomiting, diarrhea, or other complaints. The child has no significant past medical history other than the recurrent otitis media and is up-to-date on her vaccines. She has not traveled recently and has no known ill contacts but did recently start pre-school.
On physical examination, she is alert and nontoxic with a pulse of 143 bpm, temperature of 102.4° F, and normal pulse oximetry and respiratory rate. Her head and neck appear normal exteriorly, and she has no rhinorrhea, stridor, drooling, trismus, or hoarseness. On oropharyngeal examination, the mucosa and tongue are moist and the tonsils are noted to be without exudate but appear a bit inflamed and enlarged. In addition, multiple red spots are noted, primarily on the soft palate (see image). There are no ulcers observed. Her neck demonstrates enlarged anterior cervical nodes but is supple. Her lungs are clear, and her heart is rate elevated but regular without murmur. The abdomen is benign without appreciable tenderness. Her extremities are without edema or rash, and capillary refill is brisk. A screening neurologic examination is conducted due to the headache and is normal.
Which diagnostic testing should you order?