**Question:** A 3-month-old child presents with biphasic stridor and barking cough. All are true except

A. Atypical symptoms for a 3-month-old infant
B. Stridor is usually present throughout the day and night
C. Biphasic stridor implies a chronic condition
D. The child is unlikely to have a cardiac cause for stridor and cough

**Core Concept:**

In infants, the clinical presentation of respiratory symptoms is crucial in identifying the underlying causes. Biphasic stridor, characterized by two distinct phases (on inspiration and expiration), is commonly associated with upper airway obstruction, particularly in cases of foreign body aspiration, e.g., choking or aspiration pneumonia, and less commonly with lower airway obstruction, such as bronchiolitis or bronchopneumonia. Barking cough is a characteristic symptom of bronchiolitis and is caused by the inflammation and edema of the small airways (bronchioles) that result in increased work of breathing and inspiratory stridor.

**Why the Correct Answer is Right:**

In a 3-month-old infant, the biphasic stridor and barking cough are typical signs of bronchiolitis, which is an infectious respiratory illness caused by the respiratory syncytial virus (RSV). This condition primarily affects infants and young children, causing inflammation and edema in the small airways, leading to increased work of breathing and inspiratory stridor (raised, harsh, and inspiratory stridor). The barking cough is a result of the inflammation and edema of the small airways, which impairs the airflow and causes a characteristic inspiratory barking or crowing sound.

**Why Each Wrong Option is Incorrect:**

A. Atypical symptoms for a 3-month-old infant: Biphasic stridor and barking cough are common in this age group, indicating a respiratory illness.
B. Stridor is usually present throughout the day and night: Stridor in bronchiolitis may be present at any time, but the infant may experience periods of improvement or worsening due to the disease course.
C. Biphasic stridor implies a chronic condition: The term "biphasic" refers to the inspiratory and expiratory phases of stridor, not the duration of the condition. Bronchiolitis is typically acute and self-limiting, and the term "biphasic" refers to the stridor pattern, not the duration of the disease.
D. The child is unlikely to have a cardiac cause for stridor: While cardiac causes for stridor exist, bronchiolitis is more common in infants, and cardiac causes are less likely in this age group.

**Clinical Pearl:**

Bronchiolitis is a common respiratory illness in infants, predominantly caused by respiratory syncytial virus (RSV). The disease presents with biphasic stridor (inspiratory and expiratory) and barking cough due to airway inflammation and edema in the small airways (bronchioles). The treatment typically includes supportive care, oxygen therapy, and sometimes bronchod