Which metabolic process is most likely impaired in this patient? A 5-year-old boy is brought to the physician by his mother because of a 3-month history of headache, clumsiness, and fatigue. He had a generalized tonic-clonic seizure 1 month ago. His mother says that he often falls while running. Ophthalmologic examination shows ptosis of the left eye and bilateral external ophthalmoplegia. Physical examination shows weakness of the left lower extremity. Serum studies show an increased lactate concentration. Analysis of a calf muscle biopsy specimen obtained from the left lower extremity shows ragged red fibers.