Final answer:
A pregnancy test is required before obtaining a refill for isotretinoin due to its potential harm to a developing fetus.
Step-by-step explanation:
The nurse should advise the client that a Pregnancy test is required before obtaining a refill for isotretinoin. Isotretinoin is a teratogenic medication, meaning it can cause harm to a developing fetus if taken during pregnancy. Therefore, it is crucial to verify that the client is not pregnant before continuing the medication.