A 32-year-old presents to the psychiatric mental health nurse practitioner following four brief episodes of intense anxiety occurring at home and work over the last two months. He reports the episodes "come out of the blue." During the episodes, he experiences dyspnea, heart palpitations, chest pain, tingling in his lips and fingers, and sweating. His symptoms are so severe he fears he is going to die. His primary care physician has ruled out a medical cause for the episodes. He has cancelled several work meetings out of fear that he might have another attack. The client meets DSM criteria for which listed diagnosis?