A 48-year-old right-handed man was admitted with a 3-day history of brief generalised tonic-clonic seizures,muscle weakness, a 3-month history of weight loss with increasing difficulty in eating and a 1-week history of word finding difficulties and speech apraxia with reduced fine motor skills.He had a diagnosis of type 2 diabetes mellitus, epilepsy and a history of a left temporal lobe infarct 3 years ago. MRI of the brain showing bilateral coical-based signal abnormality with associated oedema, with corresponding diffusion hyperintensity. MR spectroscopy showing a lactate doublet peak. HPE of muscle biopsy was shown below.What is the most probable diagnosis?