A 46-year-old man with schizophrenia becomes restless and confused two days after his haloperidol dose was increased. He is febrile at 39.8°C, has widespread muscle stiffness, a pulse rate of 128/min and a blood pressure of 168/96 mmHg. His creatine kinase is 11 200 units/L (reference range 30–200 units/L), and his urine is dark. He has not taken antidepressants, stimulants or illicit drugs. Which one of the following is the most likely diagnosis?