A 28-year-old woman with a history of recurrent rhabdomyolysis triggered by prolonged fasting and strenuous exercise presents with acute muscle pain and myoglobinuria. Serum CK is markedly elevated. Genetic testing suggests a defect in carnitine palmitoyltransferase II (CPT II). Which investigation is most specific for confirming impaired fatty acid oxidation capacity in skeletal muscle?

Explanation

## Diagnosis of CPT II Deficiency and Muscle Fatty Acid Oxidation ### Clinical Context CPT II deficiency is a disorder of the carnitine shuttle system affecting long-chain fatty acid entry into mitochondria for β-oxidation. The myopathic form (adult-onset) presents with recurrent rhabdomyolysis triggered by fasting, exercise, or infection — exactly this patient's profile. The defect impairs the muscle's ability to oxidize fatty acids during energy demand. ### Why Muscle Biopsy with Enzyme Assay is Definitive **Key Point:** Muscle biopsy with direct measurement of CPT II enzyme activity is the gold-standard confirmatory test for CPT II deficiency when genetic testing suggests the diagnosis. **High-Yield:** Muscle biopsy provides: 1. **Histochemistry:** Lipid accumulation (Oil Red O stain) in type I (oxidative) fibres 2. **Enzyme assay:** Direct quantification of CPT II activity; severely reduced or absent in deficiency 3. **Electron microscopy:** Lipid droplet accumulation in mitochondria 4. **Confirmation of the biochemical defect** in the tissue where the clinical problem occurs This is the tissue-specific diagnostic gold standard, especially when genetic testing is equivocal or to assess residual enzyme activity. ### Comparison with Other Investigations | Investigation | Utility | Limitation | |---|---|---| | **Muscle biopsy + CPT II assay** | Direct enzyme measurement in affected tissue; confirms diagnosis | Invasive; requires specialized histology lab | | Ischemic forearm exercise test | Assesses glycolytic capacity; shows blunted lactate rise in FAO defects | Non-specific; does not identify CPT II deficiency; contraindicated in severe myopathy | | Serum carnitine/acylcarnitine | May be normal in CPT II deficiency (carnitine is not deficient) | Does not detect CPT II enzyme deficiency; not tissue-specific | | Urinary myoglobin | Confirms rhabdomyolysis; non-specific | Reflects muscle damage, not the underlying metabolic defect | **Clinical Pearl:** In CPT II deficiency, serum carnitine and acylcarnitine profiles are often **normal** because the defect is in the enzyme, not carnitine availability. This distinguishes CPT II deficiency from primary carnitine deficiency and makes muscle biopsy essential for diagnosis. **Mnemonic — CPT II Diagnosis:** **BIOPSY** = **B**iochemical **I**dentification **O**f **P**rotein **S**ystem **Y**ield (tissue-specific enzyme assay).