A 41-year-old woman presents with sudden right arm weakness and difficulty speaking. Her partner saw rhythmic jerking of the right hand for 90 seconds followed by several minutes of confusion; the weakness and speech disturbance are now resolving. She has no headache, her capillary glucose is 6.2 mmol/L (reference range 3.5–7.8), and brain imaging shows no haemorrhage or established infarction. Which one of the following is the most likely diagnosis?