Which of the following negative effects of anaphylaxis will be the most rapidly fatal if not treated immediately?

The most rapidly fatal effect of anaphylaxis among the options provided is upper airway swelling. This condition, known as angioedema, can lead to airway obstruction in a very short period of time. As the airway narrows due to swelling, it becomes increasingly difficult for the patient to breathe, which can result in respiratory failure.

Because the airway is critical for oxygenation and survival, any obstruction needs to be addressed immediately. If not treated with interventions such as epinephrine or advanced airway management, a person experiencing severe upper airway swelling can deteriorate quickly, possibly leading to death in minutes.

Other options, while also serious, do not pose an immediate threat to life in the same way. Severe hypotension (low blood pressure) can lead to shock, and systemic vasodilation contributes to this effect but typically takes longer to become fatal without intervention. Diffuse urticaria, or widespread hives, is uncomfortable and can indicate a systemic allergic reaction, but it does not directly threaten the airway or result in rapid death like upper airway swelling does.