Final answer:
Epiglottitis is the croup syndrome considered a true pediatric emergency due to the rapid onset of airway obstruction, requiring immediate attention to prevent respiratory failure.
Step-by-step explanation:
The croup syndrome considered a true pediatric emergency is epiglottitis. This is a life-threatening condition where the tissue that shields the windpipe swells, potentially blocking air to the lungs.
It is in contrast to conditions like viral croup, which is less severe, and pertussis or Respiratory Syncytial Virus (RSV), which, while serious, especially in infants, do not typically present with such acute, rapidly worsening symptoms requiring immediate airway management as epiglottitis does. Prompt recognition and management in a controlled setting like an emergency department is crucial for a child presenting with epiglottitis.