Final answer:
The nurse should teach the patient to avoid mixing more than one type of insulin in a syringe.
Step-by-step explanation:
The nurse should teach the patient to avoid mixing more than one type of insulin in a syringe. This is because different types of insulin can have different effects and mixing them together may alter their effectiveness. It is important for the patient to keep track of their insulin regimen and ensure they are administering the correct type and dose of insulin.