MELAS — Mitochondrial Maternal Inheritance

A 22-year-old woman presents with stroke-like episodes not confined to vascular territories, migraine, short stature, and sensorineural hearing loss. Serum lactate is elevated at rest and rises further with exercise. MRI shows a parieto-occipital lesion crossing vascular boundaries. Her mother and maternal aunt have diabetes and deafness; her father's side is unaffected. Muscle biopsy shows ragged-red fibers on Gomori trichrome stain. The pedigree pattern (marked **B**) demonstrates that all children of affected females are affected to varying degrees, while children of affected males are unaffected. Which of the following genetic mutations is most likely responsible for this clinical presentation?