**Core Concept**
The infant's presentation with fever, persistent cough, and pneumonia on chest x-ray suggests a respiratory infection. The most likely cause is a pathogen that commonly affects infants and young children, taking advantage of their immature immune system.

**Why the Correct Answer is Right**
Respiratory Syncytial Virus (RSV) is the leading cause of pneumonia and bronchiolitis in infants worldwide. It is highly contagious and spreads through droplet transmission. RSV infects the ciliated epithelial cells of the respiratory tract, leading to inflammation, edema, and increased mucus production. This results in symptoms such as fever, cough, and difficulty breathing, characteristic of pneumonia.

**Why Each Wrong Option is Incorrect**
**Option A:** *Streptococcus pneumoniae* is a common cause of pneumonia in children, but it is more typically seen in children over 2 years of age, and is often associated with bacterial meningitis or otitis media.

**Option B:** *Haemophilus influenzae* type b (Hib) is a common cause of pneumonia in young children, but the widespread use of Hib vaccination has significantly reduced its incidence.

**Option C:** *Pseudomonas aeruginosa* is a Gram-negative bacterium that can cause pneumonia, particularly in immunocompromised patients or those with cystic fibrosis. However, it is less common in otherwise healthy infants.

**Clinical Pearl / High-Yield Fact**
RSV infection is highly contagious and can be spread through close contact with an infected individual, contaminated surfaces, and airborne transmission.

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