Final answer:
The nurse should report the elevated BUN and creatinine levels to the health care provider, suggesting kidney function issues potentially linked to the patient's psychiatric symptoms.
Step-by-step explanation:
When a patient presents to the emergency department with mixed psychiatric symptoms and lab results show elevated Blood Urea Nitrogen (BUN) and creatinine, it indicates a potential issue with kidney function. Given these lab findings, the next best action for the nurse would be to report the findings to the health care provider. This step is crucial as it ensures that a thorough medical evaluation is conducted, potentially identifying underlying renal issues contributing to the patient's psychiatric symptoms.