A 3-year-old boy from rural Maharashtra presents with recurrent episodes of hypoglycemia, lethargy, and hepatomegaly. His mother reports that symptoms worsen during fasting periods of >6 hours and improve with frequent feeding. On examination, he is afebrile with mild hepatomegaly (3 cm below costal margin). Laboratory investigations show: blood glucose 45 mg/dL (fasting), serum ketones <0.5 mmol/L (inappropriately low for the degree of hypoglycemia), ammonia 180 µmol/L (elevated), and normal liver transaminases. Plasma carnitine is low. Which of the following is the most likely enzymatic defect?

Explanation

## Clinical Presentation Analysis **Key Point:** The combination of hypoketotic hypoglycemia (inappropriately low ketones for severe hypoglycemia), elevated ammonia, hepatomegaly, and low plasma carnitine is pathognomonic for CPT I deficiency. ### Pathophysiology of CPT I Deficiency CPT I catalyzes the first committed step in fatty acid oxidation—the carnitine-dependent transport of long-chain fatty acyl-CoA into mitochondria. When deficient: 1. **Blocked β-oxidation** → impaired ketone body production (hypoketotic hypoglycemia) 2. **Impaired energy generation** → hypoglycemia during fasting 3. **Accumulation of acyl-CoA in cytoplasm** → shunting into esterification pathways → hepatic steatosis and hepatomegaly 4. **Impaired ureagenesis** (requires mitochondrial acetyl-CoA) → hyperammonemia 5. **Carnitine sequestration** in tissues → low plasma carnitine ### Differential Diagnosis Table | Feature | CPT I | CPT II | Acyl-CoA DH | 3-Ketoacyl-CoA Thiolase | |---------|-------|--------|-------------|------------------------| | **Ketone bodies** | ↓ (hypoketotic) | ↓↓ (severe) | ↓ | ↑ (hyperketotic) | | **Ammonia** | ↑ | Normal | Normal | Normal | | **Plasma carnitine** | ↓ | Normal | Normal | Normal | | **Hepatomegaly** | Marked | Mild | Marked | Mild | | **Muscle involvement** | Absent | Present (myalgia, rhabdo) | Present | Absent | | **Trigger** | Fasting | Fasting + exercise | Fasting | Fasting | **High-Yield:** CPT I deficiency is the ONLY fatty acid oxidation disorder that presents with **hypoketotic hypoglycemia + hyperammonemia**. The hyperammonemia occurs because ureagenesis requires mitochondrial acetyl-CoA (from β-oxidation), which cannot be generated. ### Clinical Pearl CPT I deficiency is often misdiagnosed as Reye syndrome or non-alcoholic fatty liver disease (NAFLD) in children. The key discriminator is the **inappropriately low ketone level** despite severe hypoglycemia—in normal fasting, ketones should be elevated to compensate for low glucose. ### Management Implications - **Avoid fasting** (frequent carbohydrate feeding) - **Avoid high-fat diet** (cannot be oxidized) - **Supplement medium-chain triglycerides (MCTs)** (bypass CPT I, enter mitochondria directly) - **Carnitine supplementation** (does not correct the enzyme defect but may help in CPT II deficiency) [cite:Lehninger Principles of Biochemistry Ch 21]