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Answer: B — Bronchiolitis
Why this is the correct answer:
The clinical picture is classic for bronchiolitis:
| Feature | Significance |
|---|---|
| Age: 3 months | Bronchiolitis is the #1 lower respiratory tract infection in infants < 2 years, peak at 2–6 months |
| Cough + wheeze | Caused by small airway (bronchiolar) inflammation and obstruction |
| Tachypnea | Sign of increased work of breathing |
| Feeding difficulty | A hallmark feature — tachypnea interferes with coordinated suck-swallow-breathe |
| SpO₂ 90% | Indicates hypoxia from ventilation-perfusion mismatch in obstructed bronchioles |
Why the other options are wrong:
- A — Bronchial asthma: Extremely unlikely at 3 months. Asthma is rare under 1 year and essentially never diagnosed at this age.
- C — Pneumonia: Would typically present with fever, focal crepitations, and consolidation on chest X-ray — not the diffuse wheeze/hyperinflation pattern seen here. Feeding difficulty is less characteristic.
- D — Foreign body aspiration: Usually presents in toddlers (> 6 months, typically 1–3 years) who can grasp and mouth objects. A 3-month-old cannot self-introduce a foreign body.
Key teaching point:
Bronchiolitis is most commonly caused by RSV (Respiratory Syncytial Virus). The triad of wheeze + tachypnea + feeding difficulty in an infant < 2 years = bronchiolitis until proven otherwise. SpO₂ < 92–94% is a criterion for hospitalization.
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