Final answer:
After ensuring a patent airway and oxygenation, IV fluids should be administered first to a patient with full-thickness burns over 20% of their body to prevent dehydration and organ failure.
Step-by-step explanation:
A nurse in an emergency department caring for a client with full-thickness burns over 20% of their total body surface area should prepare to administer IV fluids first after ensuring a patent airway and administering oxygen. Fluid resuscitation is critical in burn patients to offset dehydration and prevent renal and circulatory failure due to the massive loss of fluids through the damaged skin. Following fluid administration, other treatments such as analgesia, antibiotics, and tetanus toxoid can be considered based on the patient's needs and the severity of the burns.