First, stridor is a high-pitched sound from narrowed airways. In infants, common causes include congenital issues or infections. Intermittent stridor suggests something that comes and goes.

Laryngomalacia is the most common cause in infants. It's when the laryngeal cartilage is floppy, causing the epiglottis to collapse into the larynx during inspiration. This leads to stridor, especially when the child is active or lying on their back. It usually improves by 1-2 years as the larynx matures.

Other options could be subglottic stenosis, but that's less common and often associated with congenital heart defects. Tracheomalacia might cause stridor too, but it's more chronic and progressive. Laryngeal webs are rare and present with more severe symptoms.

So the correct answer should be laryngomalacia. The key points are age, intermittent nature, and typical presentation. The clinical pearl is remembering that laryngomalacia is the most common cause in this age group.

**Core Concept**
Stridor in infants is typically caused by laryngomalacia, a congenital laryngeal anomaly where floppy epiglottic tissue collapses during inspiration. This is the most common cause of stridor in infants under 12 months, often resolving spontaneously as laryngeal cartilage matures.

**Why the Correct Answer is Right**
Laryngomalacia occurs due to underdeveloped laryngeal cartilage, leading to inward collapse of the epiglottis and aryepiglottic folds during inspiration. The stridor is intermittent, worsening with activity or supine positioning. It is usually benign, with symptoms peaking at 2-4 months and resolving by 18-24 months. Diagnosis is clinical, confirmed by flexible laryngoscopy showing the characteristic "omega-shaped" epiglottis.

**Why Each Wrong Option is Incorrect**
**Option A:** Subglottic stenosis is congenital but presents with continuous stridor and often associated with congenital heart disease.
**Option B:** Tracheomalacia causes inspiratory and expiratory stridor, but it is less common and often associated with chronic respiratory issues.
**Option C:** Laryngeal web is rare, presents with severe inspiratory stridor, and often requires surgical intervention.

**Clinical Pearl / High-Yield Fact**
Remember the "rule of 3s" for laryngomalacia: presents at 3 months, peaks at 6 months, and resolves by 18 months. Differentiate from other causes by the intermittent, positional nature of stridor and absence of cyanosis or feeding issues in mild cases.

**Correct Answer: C. Laryngomalacia**