Mitochondrial Disorders And Energy Failure
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USMLE Step 1 › Mitochondrial Disorders And Energy Failure
A newborn with mitochondrial DNA mutation has hypotonia and cardiomyopathy with lactate 9.0 mmol/L (0.5–2.2). The clinician explains that oxidative phosphorylation occurs across the inner mitochondrial membrane and depends on a proton gradient. Which of the following mechanisms best explains decreased ATP production in this disorder?
Defective lysosomal degradation of mitochondria causing ATP excess and reduced anaerobic glycolysis
Failure of electron transport chain complexes to pump protons, reducing proton-motive force for ATP synthase
Increased peroxisomal beta-oxidation generating excess ATP, leading to reactive oxygen species and cardiomyopathy
Decreased cytosolic glycolysis from phosphofructokinase deficiency causing low lactate and exercise intolerance only
Increased activity of pyruvate carboxylase, diverting pyruvate to oxaloacetate and increasing gluconeogenesis
Explanation
This question tests understanding of mitochondrial disorders and their impact on energy production. Mitochondrial disorders often involve defects in oxidative phosphorylation leading to varied systemic manifestations. In this vignette, the presence of neonatal hypotonia, cardiomyopathy, elevated lactate, and mitochondrial DNA mutation highlights mitochondrial dysfunction. The correct answer, A, accurately reflects the underlying pathophysiology and clinical presentation as described. The distractor, B, is incorrect due to a common misunderstanding of pyruvate carboxylase deficiency which causes lactic acidosis but not cardiomyopathy. Teaching strategies include emphasizing the genetic basis of mitochondrial disorders and the importance of correlating clinical symptoms with laboratory findings. Encourage students to focus on key vignette details to differentiate between similar metabolic disorders.
A 10-year-old female with suspected mitochondrial disease has elevated lactate and a lactate peak on MR spectroscopy. MRI shows symmetric basal ganglia lesions. Mitochondrial oxidative phosphorylation defects impair ATP generation in neurons. Which of the following mechanisms best explains the MRI lesion distribution?
Vascular occlusion of middle cerebral artery branches causing unilateral cortical infarcts and focal deficits
Copper deposition in basal ganglia due to ATP7B mutation causing low ceruloplasmin and liver disease
Immune-mediated demyelination targeting periventricular white matter causing ovoid lesions and oligoclonal bands
Selective vulnerability of high-energy brain regions to ATP depletion causing necrosis in basal ganglia and brainstem
Accumulation of very-long-chain fatty acids causing demyelination predominantly in parieto-occipital white matter
Explanation
This question tests understanding of mitochondrial disorders and their impact on energy production. Mitochondrial disorders often involve defects in oxidative phosphorylation leading to varied systemic manifestations. In this vignette, the presence of developmental delay, elevated lactate, symmetric basal ganglia lesions, and lactate peak on MR spectroscopy highlights mitochondrial dysfunction. The correct answer, A, accurately reflects the underlying pathophysiology and clinical presentation as described. The distractor, B, is incorrect due to a common misunderstanding of multiple sclerosis which shows asymmetric lesions. Teaching strategies include emphasizing the genetic basis of mitochondrial disorders and the importance of correlating clinical symptoms with laboratory findings. Encourage students to focus on key vignette details to differentiate between similar metabolic disorders.
A 40-year-old female has ptosis, ophthalmoplegia, and mild proximal weakness. Lactate is 3.4 mmol/L (0.5–2.2). Muscle biopsy shows ragged red fibers, indicating abnormal mitochondrial proliferation. Mitochondria generate ATP via oxidative phosphorylation; defects increase NADH and favor lactate formation. Which of the following laboratory findings is most indicative of mitochondrial dysfunction?
Elevated homocysteine with normal methylmalonic acid reflecting folate deficiency and megaloblastic anemia
Elevated creatine kinase above 10,000 U/L reflecting acute rhabdomyolysis from crush injury
Elevated lactate with elevated lactate:pyruvate ratio reflecting impaired electron transport chain activity
Low serum ceruloplasmin with elevated hepatic copper reflecting Wilson disease and basal ganglia injury
Elevated very-long-chain fatty acids reflecting peroxisomal beta-oxidation defect and demyelination
Explanation
This question tests understanding of mitochondrial disorders and their impact on energy production. Mitochondrial disorders often involve defects in oxidative phosphorylation leading to varied systemic manifestations. In this vignette, the presence of ptosis, ophthalmoplegia, ragged red fibers, and elevated lactate highlights mitochondrial dysfunction. The correct answer, A, accurately reflects the underlying pathophysiology and clinical presentation as described. The distractor, B, is incorrect due to a common misunderstanding of elevated CK in muscular dystrophies, not mitochondrial disorders. Teaching strategies include emphasizing the genetic basis of mitochondrial disorders and the importance of correlating clinical symptoms with laboratory findings. Encourage students to focus on key vignette details to differentiate between similar metabolic disorders.