A 4-month-old infant presents with inspiratory stridor that worsens when supine. What is the most likely diagnosis?

The presentation of inspiratory stridor in a 4-month-old infant that worsens when the infant is placed in a supine position is most consistent with laryngomalacia. This condition involves softening of the laryngeal cartilages, leading to a collapse of the airway during inhalation.

In infants with laryngomalacia, the symptoms are typically exacerbated when the child is lying flat, as this position can increase the likelihood of airway collapse. Stridor is usually audible during inhalation and may be associated with other signs such as poor weight gain or feeding difficulties, although many infants with laryngomalacia thrive despite the stridor.

While laryngotracheobronchitis (often referred to as croup) can cause stridor, it typically presents with other symptoms such as a preceding viral illness and is often seen in older children. Vocal cord paralysis could present with stridor, but it is less common and often associated with other findings such as weak cry or feeding difficulties, and the stridor may not necessarily worsen in the supine position. Tracheomalacia generally causes wheezing or expiratory stridor, not primarily inspiratory stridor, and may not show this positional worsening character.

In summary