A 34-year-old woman presents with anxiety, poor sleep, nausea and a sense that people are watching her. She stopped taking alprazolam 72 hours ago after using 6 mg daily for several months; her pulse is 108/min and she has a fine tremor, but her temperature is normal. She has no diarrhoea, inducible clonus, muscle rigidity, mydriasis or recent use of stimulants. Which one of the following is the most likely diagnosis?