A 28-year-old woman with major depressive disorder has been on sertraline 100 mg daily for 3 weeks. She presents to the emergency department with a 6-hour history of agitation, tremor, hyperthermia (39.2°C), muscle rigidity, and hyperreflexia. Her husband reports she started taking over-the-counter cough syrup containing dextromethorphan yesterday for a cold. On examination, she is diaphoretic, has clonus in both lower limbs, and her creatine kinase is 1200 U/L. What is the most likely diagnosis?