MS-1584

A 6-year-old girl is brought to the Emergency Department by her mother after she began experiencing increasing dyspnoea.

On examination, she is cyanotic, drooling, and refuses to lie back, opting instead to crouch forwards with one arm out.

A lateral X-ray radiograph of the neck displays thumb sign.

What is the most likely causative pathogen in this presentation, given the most likely diagnosis?