Metabolism & Bioenergetics MCAT Practice Question
A 34-year-old woman with a history of progressive fatigue and heat intolerance presents to her physician. Laboratory studies reveal elevated resting metabolic rate and thyroid function tests are normal. A muscle biopsy is obtained and isolated mitochondria are studied in vitro. Oxygen consumption is measured under three conditions: State 2 (substrate alone, no ADP), State 3 (substrate + ADP present), and State 4 (substrate + ADP fully consumed). The respiratory control ratio (State 3 oxygen consumption ÷ State 2 oxygen consumption) is 1.9, significantly lower than the normal range of 5-7. Which of the following best explains this abnormal respiratory control ratio?
Answer choices
- AMutation in the gene encoding Complex I NADH dehydrogenase
- BDefective ATP synthase unable to couple proton gradient to ATP production
- CUncontrolled dissipation of the proton gradient across the inner mitochondrial membraneCorrect answer
- DImpaired entry of ADP into the mitochondrial matrix via the adenine nucleotide translocase
- EDepletion of NAD+ cofactor limiting continued oxidation of substrate
- FInhibition of the citric acid cycle enzyme isocitrate dehydrogenase
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