Final answer:
The oncology nurse must stop the infusion immediately upon observing extravasation during vincristine administration, then follow the facility’s protocol for extravasation management. Therefore, the correct option is b.
Step-by-step explanation:
The first action by the oncology nurse when observing extravasation with a client receiving vincristine is to stop the infusion immediately. Vincristine is a vesicant chemotherapy medication that can cause tissue damage if it leaks out of the vein, and therefore taking prompt action is critical to minimize potential harm. After stopping the infusion, the nurse should follow the institution's extravasation protocol, which may include attempts to aspirate the drug from the site, administration of an antidote if available and indicated, and local treatment to the area such as warm or cold compresses depending on the chemotherapy agent. Documentation of the extravasation and notification of the medical team are also essential steps.