Ragged red fibers are seen in mitochondrial myopathy due to defects in which energy-producing pathway?

• A. Fatty acid oxidation
• B. Glycolysis
• C. Pentose phosphate pathway
• D. Oxidative phosphorylation

Answer: (D)

Ragged red fibers reflect a mitochondrial problem inside muscle cells. On Gomori trichrome stain, these fibers show an irregular, ragged appearance at the edges due to an excessive proliferation of mitochondria with abnormal cristae. This pattern points to defects in oxidative phosphorylation, the process that generates most cellular ATP inside mitochondria through the electron transport chain and ATP synthase. When oxidative phosphorylation is impaired, ATP production falls and mitochondria accumulate and mutate, leading to the characteristic ragged red fibers. Other pathways listed—glycolysis in the cytosol, the pentose phosphate pathway, and fatty acid oxidation—do not produce this specific mitochondrial morphology, so the best answer is oxidative phosphorylation.