A 9 month old infant presents with a 2–day history of fever, cough and breathlessness following an upper respiratory infection. She is febrile and has a respiratory rate of 80/min. Intercostal and subcostal retractions and extensive rhonchi on auscultation. A chest X–ray reveals a hyperinflated chest –

**Core Concept**
The clinical presentation suggests a lower respiratory tract infection, likely **bronchiolitis**, given the age of the patient and symptoms following an upper respiratory infection. This condition is characterized by inflammation of the small airways.

**Why the Correct Answer is Right**
The correct answer is related to the most common cause of bronchiolitis in this age group, which is **Respiratory Syncytial Virus (RSV)** infection. RSV leads to inflammation and obstruction of the small airways, resulting in symptoms such as wheezing, cough, and difficulty breathing.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while other viruses can cause similar symptoms, they are not as commonly associated with bronchiolitis in infants as RSV.
**Option B:** Incorrect as it does not directly relate to the common cause of bronchiolitis.
**Option C:** Incorrect because, although it could be a cause of respiratory symptoms, it is not the most common cause of bronchiolitis in infants.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **RSV** infection is a major cause of hospitalization in infants less than one year of age, especially during the winter months.

**Correct Answer:** D. Respiratory Syncytial Virus

✓ Correct Answer: D. Bronchiolitis