Final answer:
Agoraphobia is the phobia most often associated with panic disorder, characterized by fear of situations where escape might be difficult in case of a panic attack.
Step-by-step explanation:
The phobia most often associated with panic disorder is agoraphobia. Agoraphobia involves intense fear and anxiety about being in places where escape might be difficult or embarrassing, or where help might not be available in case of a panic attack or other embarrassing episode. Studies such as the one conducted by Kessler et al. (2006) The epidemiology of panic attacks, panic disorder, and agoraphobia have documented the close relationship between panic disorder and agoraphobia, highlighting that individuals with panic disorder may develop agoraphobia as they begin to fear the onset of future panic attacks and avoid situations they associate with these attacks.