Final answer:
For a patient diagnosed with hyperglycemic hyperosmolar syndrome (HHS), the priority nursing action is fluid and electrolyte replacement to address severe dehydration and correct osmotic imbalances. The correct answer is D).
Step-by-step explanation:
An older adult patient with type 2 diabetes has been diagnosed with hyperglycemic hyperosmolar syndrome (HHS), with a blood glucose level of 623 mg/dL. The priority nursing action in this case would be D) Fluid and electrolyte replacement. This is critical due to the severe dehydration that occurs in HHS. Insulin administration is also important, but the immediate concern is to stabilize the patient's fluids and electrolytes to reduce the high blood glucose concentration and correct osmotic imbalances.