Laryngomalacia is the most common cause of stridor in infants. It is characterized by falling of soft tissue of the larynx over the airway, thereby causing blockage and consequent stridor. The exact etiology of laryngomalacia is not known, however, a decreased muscle tone may give rise to laryngomalacia. The condition is usually self-limiting by the age of 20 months. In cases where child is unable to gain weight or thrive, surgery is indicated. Reference: https://www.childrenshospital.org/conditions-and-treatments/conditions/l/laryngomalacia Image via: https://en.wikipedia.org/wiki/Laryngomalacia
Also very common in patients with Downs syndrome