A previously healthy 8 years olf boy is brought to the emergency department by his parents because of fever and progressively worsening sore throat and dydphagia physical examination shows pharyngeal erythema with tender left and right cerviacal lymphadenopathy. Contrast enhanced computed tomography (CT) shows fluid accumulation in the retropharyngeal space. A diagnosis of retropharyngeal abscess is suspected is mostly likely involved in this process?