A mother brings her 35 year old son to the emergency department because of tremor and mutism for the past three days. His mother found him in his room this morning lying stiffly in his bed, soiled with urine and feces. He appears confused and will not respond to questions. He was diagnosed with schizophrenia last year and has been on several medications. Last month after his most recent hospital admission for schizophrenia, he was discharged with a prescription for haloperidol. On physical exam, he is visibly diaphoretic and has vital signs as follows: T 102.7, BP 140/98, P 112, R 12. His neuromuscular exam shows extremely rigid extremities, and his laboratory values are notable for a white blood cell count of 15000/mm3 and abnormally elevated creatine phosphokinase levels. What is the most likely explanation for these findings