Final answer:
The nurse's best action is to administer PRN acetaminophen to relieve the client's headache caused by isosorbide dinitrate.
Step-by-step explanation:
The nurse's best action when a client who just started taking isosorbide dinitrate (Imdur) reports a headache is to administer PRN acetaminophen. Isosorbide dinitrate is a medication used to prevent chest pain in patients with coronary artery disease and may cause headaches as a side effect. Acetaminophen is an over-the-counter pain reliever that can help alleviate the headache. The nurse should ensure that the client's overall condition is stable and monitor for any other adverse effects.