A 3-year-old child presents with recurrent episodes of hypoketotic hypoglycemia triggered by fasting or viral illness. Plasma carnitine levels are normal, and urine organic acids show elevated dicarboxylic acids. Which enzyme deficiency is most likely responsible?

## MCAD Deficiency: The Most Common Fatty Acid Oxidation Disorder ### Clinical Presentation **Key Point:** MCAD deficiency is the most frequent disorder of mitochondrial fatty acid oxidation, with an incidence of approximately 1 in 10,000–20,000 live births in developed countries. ### Diagnostic Clues in This Case | Feature | Finding | Significance | |---------|---------|---------------| | **Hypoglycemia type** | Hypoketotic (low ketones) | Impaired ketone production due to blocked β-oxidation | | **Trigger** | Fasting or viral illness | Increased demand for fatty acid oxidation | | **Carnitine levels** | Normal | Rules out primary carnitine deficiency or CPT defects | | **Urine organic acids** | Elevated dicarboxylic acids | Accumulation of medium-chain fatty acids shunted to ω-oxidation | | **Age of onset** | Early childhood (typically 6 mo–3 yr) | Classic presentation window | ### Biochemical Mechanism **High-Yield:** MCAD catalyzes the first oxidation step of medium-chain fatty acids (C6–C12). When deficient: 1. β-oxidation is blocked → **no acetyl-CoA production** → **no ketone synthesis** 2. Fatty acids accumulate and are shunted to **ω-oxidation** (peroxisomal pathway) → **dicarboxylic acids** in urine 3. Glucose production is impaired → **hypoketotic hypoglycemia** **Mnemonic: MCAD = Medium-Chain Acyl-CoA Dehydrogenase — Medium-chain fatty acids (C6–C12) are its substrate; deficiency causes Hypoketotic hypoglycemia, Dicarboxylic aciduria, and Developmental delay if untreated.** ### Why Other Enzymes Are Ruled Out ```mermaid flowchart TD A[Fatty Acid Oxidation Defect]:::outcome --> B{Carnitine levels normal?}:::decision B -->|No| C[CPT I or CPT II deficiency]:::outcome B -->|Yes| D{Urine dicarboxylic acids?}:::decision D -->|Yes| E[MCAD or LCAD deficiency]:::outcome D -->|No| F[Other β-oxidation enzymes]:::outcome E --> G{Age of onset & severity?}:::decision G -->|Early childhood, moderate| H[MCAD - most common]:::action G -->|Neonatal, severe| I[LCAD or LCHAD]:::action ``` **Clinical Pearl:** MCAD deficiency is now screened in newborn screening programs via tandem mass spectrometry (elevated C6, C8, C10 acylcarnitines). Early diagnosis and management (frequent feeds, carbohydrate supplementation during illness) prevent sudden death.