A 30-year-old man turns up with progressive confusion, then a generalised tonic–clonic seizure. MRI shows massive edema, contrast enhancement, and diffusion restriction… but it doesn’t follow any vascular territory. The neurosurgeons thought it was a tumour. Radiology thought it might be a stroke. Nobody was sure. The clues? Type 1 diabetes. Bilateral hearing loss. A family history. It was MELAS — mitochondrial encephalopathy with lactic acidosis and stroke-like episodes. A mitochondrial disorde