A 3-year-old boy from rural Maharashtra presents with recurrent episodes of hypoglycemia, hepatomegaly, and developmental delay. His parents report that symptoms worsen during fasting or illness. Serum carnitine is low (8 µmol/L, normal 20–50), and urine organic acids show elevated dicarboxylic acids. Plasma acylcarnitine profile reveals elevated long-chain acylcarnitines. What is the primary enzymatic defect in this patient?

Explanation

## Clinical Presentation & Biochemical Findings This patient presents with the classic triad of a **long-chain fatty acid oxidation disorder**: - Recurrent hypoglycemia (impaired fatty acid oxidation during fasting/illness) - Hepatomegaly (lipid accumulation in hepatocytes) - Developmental delay (neurological involvement from energy deficit) The key biochemical clues are: 1. **Elevated dicarboxylic acids in urine** — reflecting ω-oxidation of long-chain fatty acids that cannot undergo β-oxidation 2. **Elevated long-chain acylcarnitines in plasma** — hallmark of a defect in long-chain acyl-CoA dehydrogenation 3. **Low serum carnitine (8 µmol/L)** — secondary carnitine depletion due to trapping of carnitine as long-chain acylcarnitines ## Why VLCAD Deficiency? **Key Point:** Very long-chain acyl-CoA dehydrogenase (VLCAD) catalyzes the first step of mitochondrial β-oxidation for fatty acids with chain lengths of C14–C20. Deficiency leads to accumulation of long-chain acylcarnitines (C14:1-acylcarnitine is the diagnostic marker) and dicarboxylic aciduria via compensatory ω-oxidation. **High-Yield:** The combination of **elevated long-chain acylcarnitines + dicarboxylic aciduria + secondary low carnitine** is the biochemical fingerprint of VLCAD deficiency — not primary carnitine deficiency. ## Why Not the Other Options? | Feature | VLCAD Deficiency | Primary Carnitine Deficiency | CPT I Deficiency | CPT II Deficiency | |---------|------|------|------|------| | **Serum carnitine** | ↓ (secondary) | ↓↓↓ (primary, <5 µmol/L) | ↑ (elevated) | Normal/↓ | | **Urine dicarboxylic acids** | ↑↑ | Absent/minimal | ↑ | ↑ | | **Long-chain acylcarnitines** | ↑↑ (C14:1 dominant) | Normal/absent | Normal | ↑↑ | | **Hepatomegaly** | Common | Common | Common | Rare (myopathic form) | | **Inheritance** | Autosomal recessive | Autosomal recessive | Autosomal recessive | Autosomal recessive | - **Primary carnitine deficiency (SLC22A5):** Causes profoundly low carnitine (<5 µmol/L) due to renal wasting, but does NOT produce elevated long-chain acylcarnitines or significant dicarboxylic aciduria — the transport defect prevents acylcarnitine formation, not dehydrogenation. - **CPT I deficiency:** Carnitine levels are typically elevated (not low), and long-chain acylcarnitines are not elevated. - **CPT II deficiency:** Presents predominantly with myopathy/rhabdomyolysis in adults; the infantile/severe form is rare and distinct from this presentation. ## Management **Clinical Pearl:** VLCAD deficiency is managed with avoidance of fasting, a low-fat diet with medium-chain triglyceride (MCT) supplementation (MCTs bypass the VLCAD step), and emergency glucose during illness. Newborn screening via acylcarnitine profile (elevated C14:1) allows early diagnosis. [cite: Scriver's OMMBID; Saudubray JM, Metabolic Diseases 6e; Harrison's Principles of Internal Medicine 21e Ch 409]