A patient with Leigh syndrome (subacute necrotizing encephalomyelopathy) has markedly elevated blood lactate and pyruvate with a normal lactate-to-pyruvate ratio. This pattern most likely indicates deficiency of:
- A Pyruvate dehydrogenase complex
- B Complex I (NADH:ubiquinone oxidoreductase) of the ETC ✓
- C Lactate dehydrogenase
- D Alpha-ketoglutarate dehydrogenase
Correct answer: B. Complex I (NADH:ubiquinone oxidoreductase) of the ETC
Explanation
The lactate-to-pyruvate (L:P) ratio reflects the cytosolic NADH/NAD+ ratio. A normal L:P ratio with elevated lactate and pyruvate indicates both are elevated proportionally, pointing to a block in pyruvate oxidation at the level of the TCA cycle or ETC rather than pyruvate dehydrogenase. Complex I deficiency impairs NADH oxidation in the ETC, causing a 'log-jam' of reducing equivalents; excess NADH drives pyruvate to lactate by LDH, raising both but keeping the ratio near normal. PDH deficiency typically elevates pyruvate disproportionately.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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