A 5-year-old boy is brought to the outpatient clinic by his parents due to a sudden onset of high fever and difficulty swallowing. His parents report that he seemed well yesterday, but woke up this morning with a temperature of 39.5 °C and complained of a severe sore throat. He has become increasingly irritable and prefers to sit upright, leaning slightly forward. His voice has developed a "hot potato" quality, and he is visibly drooling, refusing to drink. There is no history of recent upper respiratory tract infection symptoms like rhinorrhea or cough. His vaccination records are incomplete, with no documentation of Haemophilus influenzae type b (Hib) immunization. On examination, his temperature is 39.8 °C, heart rate is 135/min, respiratory rate is 28/min, and blood pressure is 100/65 mm Hg. Oxygen saturation is 92% on room air. He exhibits mild inspiratory stridor, and his pharynx appears non-erythematous and without exudates, but direct visualization is difficult due to his distress. Considering the patient's presentation, what is the most likely diagnosis and the immediate priority for clinical intervention?