A 34-year-old woman with schizoaffective disorder becomes restless, sweaty and confused six hours after venlafaxine was increased and tramadol was prescribed for dental pain. Her temperature is 38.7°C, her pulse is 128/min, and both ankles repeatedly jerk when the feet are suddenly dorsiflexed; the upper-limb reflexes are brisk. There is no lead-pipe rigidity. Which one of the following is the most likely diagnosis?