A 4-yeаr-оld bоy develоps fever аnd аn upper respiratory infection in late December. On physical examination, the pediatrician notes whitish spots in his mouth near his second molars (see image below). A day later the boy develops a rash on his face. The rash consists of reddened macules and papules. Within the next several days, the rash becomes confluent and moves downward over his whole body. Which of the following interventions would be most helpful in confirming the diagnosis of this condition?