Diagnosis: Bronchiolitis
The most likely diagnosis is D. Bronchiolitis 1. This 7-month-old infant presents with the classic clinical constellation of viral bronchiolitis: low-grade fever, dry cough, bilateral wheezing, tachypnea (RR 47/min), and hyperinflation on chest X-ray in a child under 2 years of age 1, 2.
Clinical Reasoning
Bronchiolitis should be diagnosed based on history and physical examination alone 1. This infant demonstrates the pathognomonic features:
- Age 1-12 months: Peak age for bronchiolitis, which specifically affects children from 1 month through 23 months 2
- Viral prodrome: 3-day history of low-grade fever and cough consistent with upper respiratory infection preceding lower respiratory symptoms 3, 4
- Lower respiratory signs: Bilateral wheezing indicating small airway involvement 1
- Tachypnea: RR 47/min indicates increased work of breathing, a hallmark of bronchiolitis in infants 1-12 months 2
- Radiographic findings: Hyperinflation with infiltrates is characteristic of bronchiolitis, reflecting air trapping from small airway obstruction and inflammation 1
Why Not the Other Options
Asthma (Option A) is unlikely because:
- Asthma is extremely rare as a first presentation at 7 months of age 1
- The acute febrile illness with 3-day prodrome points to infectious etiology 1
- Asthma medications should not be used unless there is evidence of recurrent wheeze and/or dyspnea 1
Sinusitis (Option B) is incorrect because:
- This infant has clear lower respiratory tract involvement with bilateral wheezing and tachypnea 1
- Sinusitis does not cause hyperinflation on chest X-ray or bilateral wheezing 1
Influenza (Option C) is less likely because:
- While influenza can cause bronchiolitis, it represents only one of multiple viral etiologies 1, 5
- The clinical presentation is indistinguishable from RSV or other viral causes of bronchiolitis 1, 5
- The diagnosis is bronchiolitis regardless of the specific viral etiology 1
- Routine viral testing is not recommended as it does not change management 3, 4
Key Diagnostic Features of Bronchiolitis
Clinical criteria that confirm this diagnosis 1:
- Acute inflammation of small airways in a child under 2 years 1, 2
- Tachypnea, wheezing, and/or crackles following upper respiratory prodrome 1, 5
- Increased respiratory effort (this infant has tachypnea at 47/min) 2
Important Clinical Pitfalls
Do not routinely order chest X-rays for bronchiolitis 1. Radiography should be reserved for:
- Consideration of intubation 1
- Unexpected clinical deterioration 1
- Underlying cardiac or pulmonary disorder 1
This infant's chest X-ray was likely unnecessary unless there were concerns about severe disease or alternative diagnoses 1.
Risk stratification is essential: At 7 months, assess for risk factors including prematurity, chronic lung disease, congenital heart disease, or immunodeficiency, which would indicate higher risk for severe disease 1, 2.