A 2 years old boy presents with sudden onset of a cough, accompanied by clear nasal discharge. He has had mild symptoms of an upper respiratory infection for the last 2 days, but his parents became alarmed when his breathing became labored. On examination, he has a seal-like barky cough and inspiratory stridor when at rest, which worsens twitch agitation. Persistent sternal indrawing is also evident at rest. What is the most likely diagnosis?