**Core Concept**
Laryngotracheobronchitis (croup) is a common pediatric respiratory condition caused by viral infection, typically *Parainfluenza virus*, leading to inflammation of the larynx, trachea, and bronchi. It presents with a barking cough, hoarseness, and stridor, with symptom relief when the child lies down due to reduced airway obstruction from gravity and improved positioning.

**Why the Correct Answer is Right**
In croup, the inflammation causes narrowing of the upper airway, especially at the subglottic region. Stridor is often worse during inspiration and improves when the child lies down because gravity helps reduce airway compression. The condition is typically viral, self-limiting, and occurs in children aged 6 months to 3 years. The classic triad of barking cough, stridor, and hoarseness fits this presentation. Lying down reduces the severity of stridor due to improved airflow dynamics.

**Why Each Wrong Option is Incorrect**
Option A: Acute epiglottitis presents with sudden onset severe stridor, drooling, and difficulty swallowing, and is life-threatening. It does not improve with positioning and is usually associated with a fever and a "severe airway obstruction" appearance.
Option C: Foreign body aspiration may cause stridor, but it is usually sudden, unilateral, and associated with choking or coughing, not a gradual history with positional relief.
Option D: Retropharyngeal abscess causes severe neck swelling, high fever, and airway compromise, with stridor that worsens with activity or lying down, not improves.

**Clinical Pearl / High-Yield Fact**
Remember: "Croup is worse when standing, better when lying down" — this positional change is a hallmark of croup and helps differentiate it from other causes of stridor like epiglottitis or foreign body.

✓ Correct Answer: B. Laryngotracheobronchitis