Final answer:
The first thing a phlebotomist should do before entering an isolation room is to read the posted instructions to be aware of specific precautions.
Step-by-step explanation:
Before entering an isolation room, the first thing a phlebotomist should do is c. Read the posted instructions. This action ensures that they are aware of the specific precautions or infection control protocols that need to be followed for that particular patient or situation. After understanding the required procedures, the phlebotomist would typically wash hands, put on appropriate personal protective equipment (PPE) such as gloves, gown, mask, and, when appropriate, receive permission if needed.