According to the NRC, a written directive must be prepared for the administration of which radiopharmaceutical?

A written directive is a formal requirement established by the Nuclear Regulatory Commission (NRC) that mandates documentation for certain radionuclide administrations, particularly those associated with higher doses or specific therapeutic uses. In this context, the administration of 5 mCi of 131 I sodium iodide is included in these regulations because it is used primarily for therapeutic purposes in treating conditions such as hyperthyroidism and thyroid cancer.

Since the use of iodine-131 can result in significant radiation exposure to both the patient and potentially to others, it necessitates careful documentation to ensure that the procedure is justified, the dose is appropriate, and the patient is well-informed. The written directive serves as a safety measure to verify that all necessary protocols and precautions are followed, helping to coordinate care and ensure regulatory compliance.

The other options listed, while also involving radiopharmaceuticals, typically do not require a written directive under NRC guidelines due to their lower doses or diagnostic purposes rather than therapeutic use. Therefore, 5 mCi of 131 I sodium iodide is correctly identified as the scenario that requires a written directive according to NRC regulations.