Muscle histochemistry from a patient with a mitochondrial disorder

(A) Succinate dehydrogenase (SDH) stain shows mitochondrial proliferation ("ragged-blue" fibers) and strongly SDH-reactive vessels (SSVs; arrow).
(B) Cytochrome c oxidase (COX) stain shows reduced enzyme activity in individual fibers (asterisk) and a COX-negative SSV (arrow); x20.