Final answer:
When faced with a situation in which a patient is in cardiac arrest and a valid living will or DNR order cannot be located, the appropriate course of action would be to begin CPR immediately.
Step-by-step explanation:
When faced with a situation in which a patient is in cardiac arrest and a valid living will or DNR order cannot be located, the appropriate course of action would be to begin CPR immediately. CPR, or cardiopulmonary resuscitation, can help circulate blood and oxygen to vital organs until further medical assistance arrives. It is crucial to start CPR as soon as possible to improve the chances of the patient's survival.