An asymptomatic 3-yrs-old is brought to you because of right cheek swelling. The mother states that the patient had been in his normal state of health until 1 hour ago, when he developed right cheek swelling. The patient is afebrile. Examination is unremarkable except for the right cheek, which is erythematous but not warm to touch. On palpation of the right cheek, mildly tender, discrete, indurated masses are appreciated. Which is the most likely cause?