**Core Concept**
The question is testing the ability to identify a common viral cause of respiratory symptoms in a child. The underlying principle is the clinical presentation of a specific viral infection.

**Why the Correct Answer is Right**
The symptoms described, such as a barking cough, runny nose, and sore throat, are characteristic of croup, a common pediatric condition caused by the parainfluenza virus. The virus primarily affects the larynx and trachea, leading to inflammation and narrowing of the airways, resulting in the characteristic barking cough. The presence of a sore throat and runny nose further supports the diagnosis of croup. The fact that none of her siblings is sick makes a contagious viral cause more likely.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not specified, but it is likely to be another viral cause, such as influenza or RSV. However, the symptoms described are more typical of croup.
**Option B:** Adenovirus can cause respiratory symptoms, but the barking cough is more characteristic of croup.
**Option C:** RSV is a common cause of respiratory illness in children, but the barking cough and sore throat are not typical of RSV infection.

**Clinical Pearl / High-Yield Fact**
Croup is a common pediatric condition that can be managed in the primary care setting with corticosteroids and nebulized epinephrine. The barking cough is a classic clinical finding that should raise suspicion for croup.

**Correct Answer:** C.