In a patient with bronchiolitis caused by RSV, which of the following symptoms may indicate severe illness?

In cases of bronchiolitis caused by respiratory syncytial virus (RSV), respiratory distress is a clear indication of severe illness. This symptom can manifest in various forms, such as increased work of breathing, use of accessory muscles, retractions (pulling in of the skin between the ribs or at the base of the neck), nasal flaring, and cyanosis. These signs indicate that the patient's respiratory system is significantly compromised.

In the context of bronchiolitis, respiratory distress suggests that the airways are obstructed, leading to inadequate ventilation and gas exchange. It signals that the illness may be progressing or that the child may require more intensive monitoring or medical intervention, such as hospitalization or supplemental oxygen.

The other symptoms listed, like mild cough and clear nasal discharge, do not typically indicate severe illness and are more common in milder cases of bronchiolitis. Hemodynamic stability is important but pertains to the cardiovascular status rather than respiratory distress, which is the critical factor in assessing the severity of bronchiolitis in these patients.