A 28-year-old man with panic disorder is compared to a 35-year-old woman with social phobia (social anxiety disorder). The man reports sudden, severe episodes of chest pain and dyspnea occurring unpredictably throughout the day, leading him to visit the emergency department repeatedly. The woman experiences intense fear and avoidance of public speaking and social gatherings, but her anxiety is confined to these specific situations and does not occur when she is alone. Which feature best discriminates panic disorder from social anxiety disorder?