Correct Answer: B. Parainfulenza
Parainfluenza virus is the most common cause of croup (laryngotracheobronchitis) in children, presenting with the classic triad of low-grade fever, inspiratory stridor, and barking ("seal-like") cough. The clinical presentation in a 5-year-old with a 5-day history is pathognomonic for viral croup. Parainfluenza types 1, 2, and 3 are responsible for 75% of croup cases in India and globally. The condition is self-limited, viral in nature, and typically affects children aged 6 months to 5 years during autumn and winter months. The chest X-ray in croup classically shows the "subglottic narrowing" or "steeple sign" (narrowing of the subglottic trachea), which is a hallmark radiological finding. The barking cough results from subglottic edema and inflammation of the larynx and upper trachea. Management is supportive with dexamethasone and nebulized epinephrine in severe cases, as per IAP guidelines. Parainfluenza is a negative-sense, single-stranded RNA virus belonging to the Paramyxoviridae family.
Why the other options are wrong
A. Staphylococcus aureus — S. aureus causes bacterial croup or epiglottitis, which presents with high fever, acute onset, and drooling—not the insidious 5-day low-grade fever and barking cough seen here. Bacterial croup is rare in vaccinated populations and requires antibiotics. The clinical timeline and symptom pattern rule out this bacterial pathogen. C. Moraxella — Moraxella catarrhalis is a gram-negative diplococcus that causes otitis media, sinusitis, and respiratory tract infections in children, but it does NOT cause croup. It presents with purulent secretions and bacterial infection signs, not the viral prodrome with barking cough and stridor characteristic of croup. D. Hemophilus influenza — Haemophilus influenzae type b (Hib) causes epiglottitis, not croup. Epiglottitis presents with acute onset, high fever, drooling, dysphagia, and 'cherry-red' epiglottis on examination. Hib incidence has declined dramatically in India post-Hib vaccination. The gradual 5-day course with barking cough is inconsistent with Hib epiglottitis.
High-Yield Facts
- Parainfluenza is the most common cause of croup, accounting for ~75% of cases in children aged 6 months–5 years.
- Steeple sign on chest X-ray (subglottic narrowing) is the radiological hallmark of viral croup.
- Barking cough ('seal-like') + inspiratory stridor + low-grade fever = classic croup triad caused by parainfluenza.
- Parainfluenza is a negative-sense RNA virus (Paramyxoviridae); croup is self-limited and managed supportively with dexamethasone.
- Epiglottitis (Hib, S. aureus) presents acutely with high fever and drooling—NOT the insidious barking cough of croup.
Mnemonics
CROUP Causes (Viral > Bacterial) Cold-like prodrome → Respiratory Syncytial Virus, Other Paramyxoviruses (Parainfluenza, Measles), Upper airway edema → Parainfluenza #1. Parainfluenza causes 75% of croup; bacterial causes (Hib, S. aureus) cause epiglottitis, not croup. Stridor vs Drooling Stridor + barking cough = Croup (viral, parainfluenza). Drooling + high fever + acute = Epiglottitis (bacterial, Hib/S. aureus). This discriminates the two upper airway emergencies in children.
NBE Trap
NBE may pair bacterial pathogens (S. aureus, Hib) with upper airway obstruction to trap students who conflate croup with epiglottitis. The key discriminator is the clinical timeline (insidious 5-day viral prodrome vs. acute bacterial onset) and the barking cough (croup) vs. drooling (epiglottitis).
Clinical Pearl
In Indian pediatric practice, parainfluenza croup is seasonal (autumn–winter) and self-limited; most cases resolve with dexamethasone (0.6 mg/kg) and supportive care. Severe cases warrant nebulized epinephrine. Bacterial epiglottitis is now rare post-Hib vaccination but remains a medical emergency requiring airway management and antibiotics—a critical distinction in the exam room.
_Reference: Jawetz, Melnick & Adelberg's Medical Microbiology (Paramyxoviruses); OP Ghai Essentials of Pediatrics (Croup); Harrison's Principles of Internal Medicine Ch. 195 (Parainfluenza)_