What should be performed first in a patient showing signs of potential ectopic pregnancy and instability?

In a patient presenting with signs of a potential ectopic pregnancy and instability, the priority is to address the immediate risk to the patient's life due to possible internal bleeding or rupture. A laparotomy is the most appropriate initial intervention in this scenario because it allows for direct visualization of the abdominal organs, particularly the reproductive organs, and the opportunity to control any bleeding that may be occurring. This surgical approach can provide definitive treatment, as it allows for the removal of an ectopic pregnancy if found and any necessary interventions to stabilize the patient.

Other diagnostic procedures such as an abdominal ultrasound or culdocentesis could provide valuable information about the presence of an ectopic pregnancy but would delay definitive management in an unstable patient. Magnetic resonance imaging is not typically used in these acute situations due to its time requirements and limited role in rapidly addressing life-threatening conditions. Thus, in cases of ectopic pregnancy with instability, the immediate surgical intervention via laparotomy is the most critical and time-sensitive action.