Final answer:
The highest priority nursing action when admitting a patient to a psychiatric unit on an involuntary basis is to ensure patient safety.
Step-by-step explanation:
When admitting a patient to a psychiatric unit on an involuntary basis, the nursing action with the highest priority is ensuring patient safety. This includes taking measures to prevent self-harm, harm to others, and ensuring a safe environment. For example, the nursing staff may need to remove any potentially harmful objects from the patient's immediate surroundings and closely monitor the patient for any signs of agitation or aggression.