A 26-year-old man with schizophrenia stopped taking lorazepam prescribed for recurrent episodes of immobility and mutism. Two days later he is mute, alternates between maintaining rigid postures and resisting examination, and has a temperature of 39.2°C, pulse 132/min and blood pressure 165/95 mmHg. His creatine kinase is 2200 U/L (40–200), and he has not received an antipsychotic for three weeks. Which one of the following is the most likely diagnosis?