A 42-year-old woman with a history of severe asthma is brought to the emergency department in acute respiratory distress. She is wheezing bilaterally, SpO₂ is 82% on room air, and peak expiratory flow is severely reduced. She has received nebulized salbutamol (albuterol) twice in the past 2 hours without adequate response. Her blood pressure is 105/68 mmHg and heart rate is 118 bpm. Intravenous corticosteroids and magnesium sulfate have been initiated. The attending physician decides to administer a subcutaneous injection of an adrenergic agonist for acute bronchospasm relief. Which agent is most appropriate?