Croup

A 2-year-old girl is brought to the emergency department by her parents for evaluation of increased work of breathing. According to her parents, the patient began experiencing ongoing sneezing, runny nose, and watery eyes around three-days ago. This morning, she developed a cough and difficulty breathing. The patient was born prematurely at 34-weeks, but past medical history is otherwise noncontributory. Temperature is 37.5°C (99.5°F), blood pressure is 92/60 mmHg, and pulse is 110/min. Physical examination is notable for a barking cough, inspiratory stridor, and accessory muscle use during respiration. A neck radiograph is obtained and demonstrates the following finding:



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Which of the following pathogens is most likely responsible for this patient’s disease process?