A 37-year-old man repos that after a sudden financial loss, he had a bout of illness in which he felt sad and nervous and fearful, could not sleep or eat properly. He also began to hear voices telling him that he was useless and should kill himself. His symptoms stopped after approximately one week. He has had no similar episodes. Medical history includes no psychiatric conditions. Physical examination shows no abnormalities, and results of laboratory studies are within normal limits. Which of the following is the most likely diagnosis?