mrcem2-1780

A 4-year-old girl presents with stridor and a barking cough. Her mother reports that she has had a slight cold for a few days and her voice had been hoarse. Her vital signs are as follows: temperature 38.1°C, heart rate 135, respiratory rate 30, oxygen saturation 97% on room air. Her chest examination is unremarkable, but you observe the presence of stridor at rest.

What is the SINGLE most probable causative organism?