Final answer:
The first question a nurse should ask before administering a contrast medium is about allergies to iodine or shellfish to prevent any potential allergic reactions.
Step-by-step explanation:
When preparing to administer a contrast medium, the first question a nurse should ask the client is, "Are you allergic to iodine or shellfish?" This is critical to ensure the safety of the patient, as contrast media often contain iodine, which is also present in shellfish. People with allergies to iodine or shellfish may have a higher risk of an allergic reaction to the contrast medium. This is part of critical thinking in medical applications, as knowing the patient's allergies can prevent adverse reactions. Contrast mediums are used in various diagnostic tests, such as imaging of the thyroid with iodine, and require careful consideration of potential allergies to ensure patient safety and test accuracy.