**Core Concept**
Bronchiolitis is a common lower respiratory tract infection characterized by inflammation and obstruction of the small airways in infants. The most likely cause of bronchiolitis in a 6-month-old infant is a viral infection, specifically respiratory syncytial virus (RSV).

**Why the Correct Answer is Right**
RSV is the primary cause of bronchiolitis in infants, particularly those under 1 year of age. RSV infects the ciliated epithelial cells lining the airways, causing damage and disrupting the mucociliary clearance mechanism. This leads to the accumulation of mucus and debris, resulting in airway obstruction and the characteristic symptoms of bronchiolitis.

**Why Each Wrong Option is Incorrect**
**Option A:** Adenovirus can cause respiratory infections, but it is less commonly associated with bronchiolitis in infants compared to RSV. Adenovirus infections often present with different symptoms, such as pharyngitis or conjunctivitis.
**Option B:** Influenza virus can cause respiratory illness in infants, but it is not the most likely cause of bronchiolitis in this age group. Influenza infections often present with systemic symptoms, such as fever and malaise, in addition to respiratory symptoms.
**Option C:** Mycoplasma pneumoniae is a cause of pneumonia in older children and adults, but it is not typically associated with bronchiolitis in infants.

**Clinical Pearl / High-Yield Fact**
RSV infection is highly contagious and can spread through close contact with an infected individual. Infants with underlying medical conditions, such as heart disease or chronic lung disease, are at increased risk of severe RSV infection.

**Correct Answer: C. RSV. Respiratory Syncytial Virus.**