Which is a sign of a panic disorder according to listed symptoms?

Panic disorder is characterized by recurrent, unexpected panic attacks, which are sudden episodes of intense fear or discomfort. One of the hallmark signs of a panic attack is the presence of physical symptoms, including palpitations and a rapid heart rate. These symptoms reflect the body's acute fight-or-flight response, which can occur during a panic attack. Individuals experiencing a panic attack may also report feelings of choking, shortness of breath, dizziness, or a sense of impending doom, which can be overwhelming and distressing.

In the context of the provided options, palpitations and rapid heart rate are clearly tied to the physiological responses that occur in panic disorder, making them significant indicators of the condition. The other symptoms, while they may reflect other mental health issues or stress responses, do not specifically align with the diagnostic criteria for panic disorder in the same way. Chronic fatigue and persistent sadness may be more indicative of conditions such as chronic fatigue syndrome or depression, respectively. Difficulty concentrating can occur in a variety of disorders but does not specifically point to panic disorder as prominently as palpitations and a rapid heart rate do.