Metabolism & Bioenergetics MCAT Practice Question
A 34-year-old woman presents with progressive fatigue, dyspnea on exertion, and muscle weakness over 3 months. Laboratory studies show normal fasting glucose and lactate levels. Muscle biopsy mitochondria demonstrate normal oxygen consumption rates and intact electron transport chain enzyme complexes. However, the ATP/ADP ratio in isolated mitochondria is markedly reduced, and the proton gradient across the inner mitochondrial membrane is severely diminished compared to controls. Glycolysis proceeds at normal rates in her myocytes. Which of the following best explains these findings?
Answer choices
- AMutation in cytochrome c oxidase (Complex IV) resulting in reduced electron transport efficiency
- BDeficiency of NADH dehydrogenase (Complex I) impairing NADH oxidation
- CLoss-of-function mutation in ATP synthase or increased inner membrane permeability to protonsCorrect answer
- DInhibition of pyruvate dehydrogenase complex reducing acetyl-CoA entry into the citric acid cycle
- EReduced expression of citric acid cycle enzymes limiting NADH and FADH₂ production
- FImpaired mitochondrial import of ADP preventing ATP/ADP exchange across the inner membrane
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