Which vital sign should be monitored closely after an anaphylactic reaction?

Monitoring blood pressure is crucial after an anaphylactic reaction because this condition often leads to significant cardiovascular changes, including hypotension (low blood pressure). Anaphylaxis causes widespread vasodilation and increased vascular permeability, which can result in fluid loss and decrease in blood volume, ultimately affecting blood pressure. By closely observing the blood pressure, healthcare providers can quickly assess the patient's hemodynamic stability and determine the need for interventions, such as fluid resuscitation or administration of vasopressors, to stabilize the patient. Although other vital signs like respiratory rate and heart rate are also important indicators of a patient's condition during anaphylaxis, blood pressure is particularly critical as it reflects the body’s ability to maintain adequate perfusion to vital organs under the stress of an allergic reaction.