Final answer:
The client's rapidly beating heart and shaking hands are symptomatic of the alarm reaction stage of the general adaptation syndrome (GAS), part of the stress response known as the fight-or-flight response.
Step-by-step explanation:
A client experiencing a rapidly beating heart and shaking hands in response to an approaching diagnostic test is displaying symptoms of the stress response, particularly the alarm reaction stage of the general adaptation syndrome (GAS). This initial reaction, often referred to as the fight-or-flight response, prepares the body to deal with perceived threats. During this physiological response, hormones such as epinephrine and norepinephrine are released, leading to increased heart rate and other bodily changes that provide energy to manage the situation.