Final answer:
The priority action by the nurse is administering prescribed mood-stabilizing medications for a newly admitted client with bipolar disorder experiencing mania.
Step-by-step explanation:
The priority action by the nurse for a newly admitted client with bipolar disorder experiencing mania is administering prescribed mood-stabilizing medications. Mania is a state of elevated mood, and mood stabilizers are essential in managing the manic symptoms by regulating neurotransmitters in the brain. Although establishing a consistent routine, encouraging physical activities, and ensuring adequate nutrition are important, they are not the priority when managing acute symptoms of mania.