A 5-yrs-old girl presents with a several day history of localized swelling in the right side of her neck. There is no recent history of sore throat. Physical examination finds a low-grade fever, and one very tender, firm, slightly enlarged lymph node is palpated in the right cervical region. A CBC reveals a mild leukocytosis. A fine needle aspiration of the lymph node reveals scattered neutrophils. Which of the following is the most likely diagnosis?