Lateral X-ray of the upper airway in a 2-year-old boy reveals a “thumb sign”. The most likely diagnosis is:
**Core Concept**
The "thumb sign" on a lateral X-ray of the upper airway in a child is a radiographic marker for a specific congenital anomaly of the larynx. This sign is indicative of a posterior laryngeal cleft, a rare congenital defect characterized by an abnormal communication between the larynx and the trachea.
**Why the Correct Answer is Right**
The "thumb sign" is caused by the protrusion of the cleft into the laryngeal lumen, creating a thumb-like appearance on the X-ray. This is due to the abnormal posterior displacement of the laryngeal cartilages. The posterior laryngeal cleft is a result of incomplete fusion of the laryngeal cartilages during embryonic development. The cleft is usually located between the arytenoid cartilages and the cricoid cartilage.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the "thumb sign" or posterior laryngeal cleft.
**Option B:** Epiglottitis is a condition that involves inflammation of the epiglottis, which may cause airway obstruction, but it does not produce the "thumb sign".
**Option C:** Tracheomalacia is a condition characterized by softening of the tracheal cartilages, leading to airway collapse. It does not produce the "thumb sign" on X-ray.
**Clinical Pearl / High-Yield Fact**
The "thumb sign" is a classic radiographic finding in posterior laryngeal cleft, a rare congenital anomaly that can cause significant airway compromise in children. Recognition of this sign is crucial for prompt diagnosis and management.
**Correct Answer:** C.