A 28-year-old patient is brought to the emergency department with sudden onset of shortness of breath, widespread urticaria, and dizziness after eating shellfish. The patient's blood pressure is 80/50 mmHg, and their respiratory rate is 28 breaths per minute. What is the most appropriate initial intervention?
Give intravenous diphenhydramine
Administer intramuscular epinephrine
Start high-flow oxygen therapy
Initiate rapid IV fluid bolus