Correct answer: Thumb sign, as seen in epiglottitis. The sign is caused by a thickened free edge of the epiglottis, which causes it to appear more radiopaque than normal, resembling the distal thumb. Auscultation will reveal a stridor and inspiratory wheezing. The incidence has dropped considerably since the Haemophilus influenza B vaccine. It's a medical emergency; give penicillin IV and refer for emergency treatment with ENT and anaesthetic opinion.