ca-mccqe1 MCQ (SBA): Acute severe asthma exacerbation

A 6-year-old boy with a history of mild asthma presents to the emergency department with severe respiratory distress after a viral illness. He is speaking in single words, has marked intercostal indrawing and SpO₂ 89% on room air. Initial management includes oxygen and nebulised salbutamol plus ipratropium. According to asthma emergency care principles, which additional therapy should be administered early?