**Core Concept:** Stridor in newborns is a high-pitched, whistling sound heard during breathing, usually due to airway obstruction. Common causes include anatomical abnormalities, infections, and congenital diseases.

**Why the Correct Answer is Right:** The commonest cause of stridor in newborns is **Congenital Laryngomalacia (CL)**. Laryngomalacia is a relatively common condition characterized by the collapse of the supraglottic region, leading to airway obstruction. In CL, the weak or redundant mucosa and cartilage in the supraglottis cause the airway to collapse during inspiration, resulting in stridor.

**Why Each Wrong Option is Incorrect:**

A. **Congenital Subglottic Stenosis (CSS)** is a less common cause of stridor in newborns, involving stenosis (narrowing) of the glottis due to scarring or narrowing of the airway. CSS is not the correct answer because CL is more common.

B. **Nasogastric Tube (NGT) insertion** can lead to stridor due to inflammation or injury of the airway, but it is not the most common cause.

C. **Adenovirus infection** can cause stridor in newborns due to airway inflammation, but it is less common than CL.

D. **Hyaline cartilage deficiency** in the larynx is a rare cause of stridor and not the most common.

**Clinical Pearl:** The correct answer, Congenital Laryngomalacia, is the most common cause of stridor in newborns. Clinicians should be aware of this condition to promptly diagnose and manage the condition, ensuring timely relief of stridor and preventing complications.