Final answer:
For a responsive cardiac emergency patient with palpable pulses, vitals should be checked, and an AED applied only if the heart rate is bradycardic, following the device's protocols.
Step-by-step explanation:
The best step for a responsive cardiac emergency patient with palpable pulses is D. Check vitals and apply AED if the heart rate is bradycardic. The use of an automated external defibrillator (AED) is crucial in circumstances where cardiac arrhythmias such as ventricular fibrillation or ventricular tachycardia are present.
These devices are designed to be used by laypersons and can diagnose the patient's heart condition and deliver a shock with appropriate energy and waveform to restore a normal heartbeat. It is, however, essential to ensure that CPR is only performed when the patient has no pulse, and an AED should only be used as directed by its built-in protocols for rhythm assessment.