What condition is indicated by the "thumb sign" on a lateral x-ray?

The "thumb sign" observed on a lateral x-ray is a classic radiological finding associated with epiglottitis. This sign indicates the presence of a swollen epiglottis, which can appear enlarged and rounded, resembling a thumb on the imaging. Epiglottitis is an inflammation of the epiglottis that can lead to life-threatening airway obstruction, particularly in children and unvaccinated populations.

The swelling is usually due to a bacterial infection, most often caused by Haemophilus influenzae type b (Hib) in children, or by other organisms in adults. The lateral x-ray helps in confirming the diagnosis and assessing the degree of swelling, which is critical for determining the urgency of intervention, such as intubation or other airway management techniques.

The other options represent conditions that do not exhibit the "thumb sign" in imaging. Asthma may show hyperinflation of the lungs on imaging, a pneumothorax would typically present with a visible pleural line, and pneumonia may demonstrate consolidation or effusion, but none of these would present with the characteristic swelling of the epiglottis as seen with epiglottitis.