A 34-year-old woman from Delhi presents with recurrent episodes of muscle weakness, myalgia, and dark urine after fasting or strenuous exercise. Serum creatine kinase is markedly elevated at 8000 IU/L. Genetic testing reveals a mutation in the SUCLA2 gene encoding ADP-forming succinyl-CoA synthetase (also called succinate thiokinase), a key enzyme of the TCA cycle. She is counselled about dietary and activity modifications. What is the most appropriate next step in management?

Explanation

## Clinical Context This patient has a deficiency in succinyl-CoA synthetase, a TCA cycle enzyme catalysing the reversible conversion of succinyl-CoA to succinate with coupled GTP/ATP generation. Loss of this enzyme impairs: - ATP production from the TCA cycle - Mitochondrial energy metabolism - Exercise tolerance and fasting adaptation ## TCA Cycle Role of Succinyl-CoA Synthetase **Key Point:** Succinyl-CoA synthetase (also called succinate thiokinase) catalyses the only substrate-level phosphorylation step in the TCA cycle: $$\text{Succinyl-CoA} + \text{GDP (or ADP)} + \text{Pi} \rightarrow \text{Succinate} + \text{GTP (or ATP)} + \text{CoA}$$ This is the direct ATP-generating step; loss of this enzyme severely compromises energy production. ## Management Strategy for TCA Cycle Enzyme Deficiencies | Intervention | Rationale | Timing | | --- | --- | --- | | **Avoid fasting/intense exercise** | Prevents acute energy crisis and rhabdomyolysis | Immediate lifestyle modification | | **L-carnitine supplementation** | Enhances fatty acid oxidation and alternative ATP pathways; supports mitochondrial function | First-line pharmacotherapy | | **Frequent small meals** | Maintains glucose availability and reduces reliance on stored energy | Dietary counselling | | **Hydration & monitoring** | Prevents acute renal injury from myoglobinuria | Ongoing | **High-Yield:** In TCA cycle enzyme deficiencies, the cornerstone is lifestyle modification (avoid triggers) + L-carnitine (supports alternative energy pathways) rather than acute crisis management or invasive diagnosis. ## Why This Answer is Correct L-carnitine supplementation enhances the carnitine shuttle system, promoting fatty acid β-oxidation and acetyl-CoA entry into the TCA cycle via an alternative route. Combined with avoidance of fasting and intense exercise (which trigger acute energy depletion), this is the evidence-based first-line management for succinyl-CoA synthetase deficiency and other primary TCA cycle enzyme defects. **Clinical Pearl:** Patients with TCA cycle enzyme defects tolerate fed states and moderate activity; acute crises occur during prolonged fasting or extreme exertion when ATP demand exceeds the compromised mitochondrial supply.