Metabolism & Bioenergetics MCAT Practice Question
A 34-year-old woman presents with progressive muscle weakness, fatigue, and dyspnea on exertion over the past 6 months. Serum lactate is elevated at rest. Muscle biopsy shows ragged-red fibers. Researchers isolate mitochondria from her muscle tissue and measure oxygen consumption rates. Baseline respiration with succinate as substrate shows moderately reduced oxygen consumption compared to healthy controls. When an uncoupler (FCCP) is added to dissipate the proton gradient across the inner mitochondrial membrane, oxygen consumption in the patient's mitochondria increases dramatically and approaches normal levels. In contrast, adding the uncoupler to control mitochondria causes only a modest further increase in oxygen consumption. Which of the following best explains these findings?
Answer choices
- ADefective succinate dehydrogenase (Complex II) with reduced electron transfer capacity
- BMutation in ATP synthase subunit genes resulting in impaired ATP production despite normal proton gradient
- CExcessive proton leak across the inner mitochondrial membrane due to lipid peroxidation
- DComplete blockade of cytochrome c oxidase (Complex IV) preventing normal electron transport
- EDefective adenine nucleotide translocase preventing ADP entry into the mitochondrial matrix
- FImpaired coupling between proton gradient and ATP synthesis, creating excessive back-pressure on the electron transport chainCorrect answer
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