A 28-year-old woman with a history of recurrent myalgia and rhabdomyolysis triggered by prolonged fasting and exercise is suspected to have a mitochondrial fatty acid oxidation defect. Which investigation is most appropriate to identify the specific enzyme defect?

## Diagnosis of Mitochondrial Fatty Acid Oxidation Defects **Key Point:** Fibroblast fatty acid oxidation assay using radiolabeled palmitate is the **functional gold standard** for identifying specific enzyme defects in the β-oxidation pathway. ### Why Fibroblast Assay? #### Mechanism of the Test 1. **Substrate**: Radiolabeled palmitate (^14^C or ^3^H) — a long-chain fatty acid 2. **Measurement**: Quantifies the rate of: - Oxidation to CO~2~ (complete oxidation) - Ketone body production (acetyl-CoA → ketones) - Acyl-CoA accumulation (substrate utilization) 3. **Cell Type**: Cultured patient fibroblasts retain the genetic defect; non-invasive and renewable #### Diagnostic Pattern by Enzyme Defect | Enzyme Defect | Palmitate Oxidation | Acylcarnitine Pattern | Clinical Trigger | |---|---|---|---| | **CPT I deficiency** | ↓↓ (severe) | Normal | Fasting, exercise | | **CPT II deficiency** | ↓↓ (severe) | ↑ Long-chain | Fasting, exercise, cold | | **VLCAD deficiency** | ↓↓ (severe) | ↑ C14–C18 | Fasting, illness | | **LCHAD deficiency** | ↓↓ (severe) | ↑ C16–C18-OH | Pregnancy complications | | **Trifunctional protein (TFP) deficiency** | ↓↓ (severe) | ↑ Long-chain 3-OH | Fasting, exercise | **High-Yield:** Fibroblast assay **identifies the specific enzyme defect** and guides targeted treatment (e.g., carnitine supplementation for CPT II, dietary fat restriction for VLCAD). ### Why This Patient Fits - **Myalgia and rhabdomyolysis** triggered by fasting/exercise → classic presentation of FAO defect - **Metabolic stress** (prolonged fasting, exercise) uncovers the defect - **Need to identify the specific enzyme** → fibroblast assay is definitive **Clinical Pearl:** Patients with FAO defects present with **hypoketotic hypoglycemia** (cannot generate ketones from fat) and **myopathy** (muscle cannot oxidize fat for energy during stress). ### Workflow for FAO Defect Diagnosis ```mermaid flowchart TD A[Suspected FAO defect<br/>Myalgia + Rhabdomyolysis + Fasting trigger]:::outcome --> B[Plasma acylcarnitine profile<br/>by MS/MS]:::action B --> C{Abnormal acylcarnitine<br/>pattern?}:::decision C -->|Yes| D[Fibroblast fatty acid<br/>oxidation assay]:::action C -->|No| E[Consider other diagnosis]:::outcome D --> F[Specific enzyme defect<br/>identified]:::outcome F --> G[Genetic testing<br/>for confirmation]:::action G --> H[Targeted treatment<br/>e.g., carnitine, diet]:::action ```