A 30-year-old male presents with acute shortness of breath,…

A 30-year-old male presents with acute shortness of breath, widespread hives, and facial swelling. He denies any past medical history and takes no medications. During your assessment, you hear wheezing over all lung fields. His blood pressure is 90/50 mm Hg and his heart rate is 110 beats/min. In addition to giving him 100% oxygen, the MOST important treatment for this patient is: