A 38-year-old woman from rural Maharashtra presents with progressive fatigue, dyspnea on exertion, and recurrent syncope over 6 months. On examination, she is cyanotic and has clubbing. Investigations reveal severe lactic acidosis (pH 7.21, lactate 8.5 mmol/L), normal blood glucose, and normal liver function tests. Muscle biopsy shows ragged-red fibres. Genetic testing confirms a pathogenic variant in the MT-ND1 gene encoding NADH dehydrogenase (Complex I). Which of the following best explains the severe lactic acidosis in this patient?

Explanation

## Pathophysiology of Complex I Deficiency and Lactic Acidosis **Key Point:** Complex I (NADH dehydrogenase) catalyzes the transfer of electrons from NADH to ubiquinone, the first step of the electron transport chain. Deficiency impairs NADH reoxidation, disrupting the NAD^+^/NADH ratio. ### Mechanism of Lactic Acidosis When Complex I is non-functional: 1. **NADH accumulates** and NAD^+^ becomes depleted 2. **Glycolysis is impaired** because it requires NAD^+^ as a cofactor (lactate dehydrogenase reaction: pyruvate + NADH ↔ lactate + NAD^+^) 3. **Pyruvate cannot be efficiently oxidized** in the TCA cycle (which requires NAD^+^-dependent dehydrogenases) 4. **Pyruvate is shunted toward lactate formation** to regenerate NAD^+^ for continued glycolysis 5. **Net result:** Severe lactic acidosis despite normal glucose metabolism $$\text{NAD}^+ \text{ deficit} \rightarrow \text{Pyruvate} \rightarrow \text{Lactate} \uparrow$$ **High-Yield:** Mitochondrial disorders affecting the ETC (especially Complex I, III, IV) classically present with **lactic acidosis + normal blood glucose + normal liver function** — this triad is pathognomonic for mitochondrial cytopathy. ### Clinical Correlation **Clinical Pearl:** Ragged-red fibres on muscle biopsy indicate abnormal mitochondrial proliferation and are a hallmark of mitochondrial myopathy. The cyanosis and clubbing reflect chronic tissue hypoxia from impaired aerobic metabolism. **High-Yield:** MT-ND1 mutations cause Leber hereditary optic neuropathy (LHON) or mitochondrial myopathy—both present with lactic acidosis and bioenergetic crisis. ## Why This Answer Is Correct The fundamental defect in Complex I deficiency is **failure to reoxidize NADH**, creating a NAD^+^ deficit. This forces pyruvate into lactate formation as the only way to regenerate NAD^+^ for glycolysis to continue. The result is severe lactic acidosis with preserved glucose utilization (unlike hepatic failure or sepsis).