What is the most likely diagnosis for a 5-month-old infant with coughing, sneezing, circumoral cyanosis, and bilateral infiltrates on chest X-ray (CXR)?

Bronchiolitis

The most likely diagnosis is C. Bronchiolitis, which classically presents in infants under 1 year with cough, sneezing, rhonchi, crackles, and bilateral infiltrates on chest radiograph 1, 2.

Clinical Reasoning

Age and Presentation Pattern

• A 5-month-old infant falls squarely within the peak age range for bronchiolitis (typically 2-6 months) 3
• The 3-day prodrome of coughing and sneezing represents the typical viral upper respiratory phase that precedes lower respiratory involvement 3
• Frothy sputum around the mouth is highly characteristic of bronchiolitis, reflecting the copious secretions and small airway obstruction that define this condition 1

Physical Examination Findings

• Rhonchi throughout the chest with basal crackles and good bilateral air entry is the classic auscultatory pattern for bronchiolitis 1, 2
• The presence of circumoral cyanosis indicates hypoxemia from ventilation-perfusion mismatch, which occurs in moderate-to-severe bronchiolitis 1
• Good air entry bilaterally argues against significant consolidation or large effusion that would be expected with bacterial pneumonia 1

Radiographic Interpretation

• Bilateral infiltrates on CXR in an infant with bronchiolitis typically represent peribronchial thickening, atelectasis, and hyperinflation rather than true consolidation 4, 5
• Viral lower respiratory tract infections, including bronchiolitis, commonly produce bilateral infiltrates that can be misinterpreted as pneumonia 1, 4

Why Not the Other Options

Pneumonia (Option B)

• Bacterial pneumonia in a 5-month-old would more typically present with:
  • Higher fever and more toxic appearance 1, 3
  • Focal rather than diffuse findings on examination 1
  • Unilateral or lobar consolidation rather than bilateral infiltrates 1, 3
• The presence of sneezing and the gradual progression over 3 days favors a viral process over bacterial pneumonia 3
• Wheezing and rhonchi throughout suggest small airway disease (bronchiolitis) rather than alveolar disease (pneumonia) 2, 3

Heart Failure (Option A)

• Heart failure would typically present with:
  • Poor feeding, failure to thrive, or diaphoresis with feeds 1
  • Hepatomegaly and possibly a cardiac murmur 1
  • No preceding viral prodrome of coughing and sneezing 1
• The 3-day history of upper respiratory symptoms makes a primary cardiac etiology unlikely 3

Bronchiolitis Obliterans (Option D)

• Bronchiolitis obliterans is a chronic condition that develops weeks to months after a severe pulmonary insult (severe viral infection, toxic inhalation, or transplantation) 5
• This infant has an acute 3-day illness without any preceding severe respiratory event or risk factors 5
• The acute presentation with viral prodrome is incompatible with bronchiolitis obliterans 5

Clinical Caveats

• The term "bilateral infiltrates" on CXR in infants is often overinterpreted as pneumonia when it actually represents the peribronchial changes and atelectasis of bronchiolitis 4, 6
• Respiratory syncytial virus (RSV) is the most common pathogen in this age group and clinical scenario 3
• The presence of circumoral cyanosis warrants hospitalization for oxygen monitoring and supportive care 1, 7
• If this infant fails to improve or worsens despite supportive care, consider secondary bacterial pneumonia as a complication 1, 3