Final answer:
The nurse should first initiate cardiac monitoring due to the client's severe hypertension and heart palpitations, which can indicate a potential hypertensive crisis or cardiac event. This allows for immediate detection of any life-threatening issues.
Step-by-step explanation:
When a client presents in the emergency department with a headache, heart palpitations, a blood pressure of 210/105 mm Hg, and a high temperature of 39.9°C (103.8°F), the priority is to address the most life-threatening conditions first. With the client's blood pressure being significantly elevated, there is a risk of hypertensive crisis, which can lead to organ damage and other serious complications. Heart palpitations may signal a cardiac event, which alongside the abnormal vital signs, necessitates immediate attention.
Thus, the first action the nurse should take is to initiate cardiac monitoring for the client (option B). This action is crucial as it allows for continuous observation of the heart's activity and early detection of any potentially life-threatening dysrhythmias or ischemic changes. While administering antipyretic medication (option A) and performing a neurological assessment (option C) are important, they can be done after ensuring the client's cardiac stability. Starting an IV line for antipsychotic medication (option D) is not the initial priority and would come later after a comprehensive assessment and diagnosis.