A 3-year-old boy from rural Maharashtra is brought to the emergency department with a 2-day history of severe vomiting, lethargy, and hypoglycemia (blood glucose 35 mg/dL). His mother reports he has not eaten for 18 hours due to gastroenteritis. On examination, he is lethargic with hepatomegaly. Serum ammonia is elevated at 180 µmol/L (normal <50). Urine organic acids show elevated dicarboxylic acids. What is the most likely enzymatic defect responsible for his clinical presentation?

Explanation

## Clinical Recognition of MCAD Deficiency **Key Point:** MCAD deficiency is the most common disorder of fatty acid oxidation, presenting with hypoketotic hypoglycemia, elevated liver transaminases, and elevated dicarboxylic acids in urine during metabolic stress (fasting, infection, gastroenteritis). ### Pathophysiology of MCAD Deficiency Medium-chain acyl-CoA dehydrogenase catalyzes the first dehydrogenation step in the β-oxidation of medium-chain fatty acids (C6–C12). When deficient: 1. **Impaired fatty acid oxidation** → inability to generate acetyl-CoA and ATP during fasting 2. **Hypoketotic hypoglycemia** → glucose drops without appropriate ketone body production (ketones require adequate acetyl-CoA flux) 3. **Dicarboxylic acid accumulation** → medium-chain acyl-CoA intermediates are shunted to ω-oxidation in peroxisomes, producing dicarboxylic acids (adipic, suberic acids) that appear in urine 4. **Hepatomegaly and hyperammonemia** → fatty infiltration of liver; ammonia accumulation due to impaired TCA cycle function and reduced ATP for urea cycle ### Diagnostic Clues in This Case | Finding | Significance | |---------|-------------| | **Fasting/metabolic stress** (18 hrs, gastroenteritis) | Triggers reliance on fatty acid oxidation | | **Hypoketotic hypoglycemia** (glucose 35 mg/dL) | Hallmark of MCAD — ketones should be high but are inappropriately low | | **Elevated dicarboxylic acids in urine** | Pathognomonic for medium-chain FAO defects | | **Hepatomegaly** | Fatty infiltration from impaired β-oxidation | | **Hyperammonemia** | Secondary to hepatic dysfunction and reduced ATP | | **Age 3 years** | MCAD typically presents in early childhood during first metabolic stress | **High-Yield:** MCAD deficiency follows **autosomal recessive** inheritance and is screened in newborn screening programs (elevated C6–C10 acylcarnitines on tandem MS). Acute management: IV dextrose to suppress lipolysis and restore ATP. ### Why Ketones Are Low (Hypoketotic) Ketone body synthesis requires: - Adequate acetyl-CoA from fatty acid oxidation - Functional mitochondrial β-oxidation pathway In MCAD, fatty acids cannot be efficiently oxidized → acetyl-CoA is insufficient → ketogenesis is blunted despite fasting. This is the **defining biochemical signature** and distinguishes MCAD from other causes of hypoglycemia (e.g., insulin excess, which would also lower ketones but in a different context). **Clinical Pearl:** The combination of **hypoketotic hypoglycemia + dicarboxylic aciduria + metabolic stress** is virtually diagnostic of MCAD deficiency in a child.