A 39-year-old man with bipolar disorder is brought to the emergency department one hour after becoming agitated and incoherent. His medicines are sertraline, tramadol and recently commenced linezolid; he has not taken an antipsychotic for six months. He is sweating, has a temperature of 38.7°C, inducible ankle movements that continue after the foot is dorsiflexed, brisk reflexes and frequent diarrhoea. His muscle tone is not increased. Which one of the following is the most likely diagnosis?