Final answer:
The initial priority for the nurse is to check the client's vital signs after administering nifedipine to the wrong client, following nursing assessment protocols to ensure the client's safety before taking further administrative steps.
Step-by-step explanation:
The priority nursing action when a nurse realizes that nifedipine was administered to the wrong client is to check the client's vital signs. This initial step follows the nursing process priority-setting framework starting with assessment, before any plans or interventions are implemented. Checking vital signs is critical as nifedipine affects blood pressure and heart rate, and any deviations need to be addressed immediately. After assessing the patient and ensuring stability, subsequent steps would include notifying the charge nurse, filling out an occurrence report following institutional policies, and documenting an objective description of what transpired in the client's chart.