Final answer:
The nurse's priority action would be to document the patient's BMI of 16.1 and take steps to improve their health.
Step-by-step explanation:
The priority action for a nurse when the BMI of a patient is 16.1 would be to document the patient's BMI. This is important for keeping accurate patient records and monitoring their health. Decreasing caloric intake to 1200 calories per day and conferring with a dietitian would also be appropriate actions to help the patient improve their health and achieve a healthier BMI. Planning a return visit in one week allows the nurse to monitor the patient's progress and make any necessary adjustments to their care plan.