What should be done immediately after confirming a cardiac arrest?

Starting chest compressions immediately after confirming a cardiac arrest is critical because it helps maintain blood flow to vital organs, especially the brain and heart. Cardiac arrest leads to the cessation of effective blood circulation, which can result in irreversible damage within a few minutes. By initiating chest compressions without delay, you can provide the necessary mechanical support to circulate any available oxygenated blood, which is vital for the victim's survival until professional help arrives.

The other options do not address the immediate need for actions that can save a life in the context of cardiac arrest. Checking for allergic reactions is not relevant in this acute emergency, transporting the victim without first providing effective CPR compromises the chances of survival, and waiting for someone else to arrive delays critical care when every second counts. Thus, starting chest compressions is the most appropriate and life-saving response.