A 6-month-old Indian boy presents with severe hypoglycemia (blood glucose 35 mg/dL), hepatomegaly, and lactic acidosis. His parents report seizures after 3–4 hours of fasting. Which is the most common enzyme deficiency responsible for this presentation?

Explanation

## Enzyme Deficiency in Severe Fasting Hypoglycemia with Hepatomegaly and Lactic Acidosis ### Clinical Case Analysis The constellation of: - **Severe hypoglycemia** (35 mg/dL) after short fasting (3–4 hours) - **Massive hepatomegaly** - **Lactic acidosis** - **Age 6 months** (typical presentation age) ...is pathognomonic for **Type I glycogen storage disease (von Gierke disease)** caused by **glucose-6-phosphatase deficiency**. ### Pathophysiology of Glucose-6-Phosphatase Deficiency **Key Point:** Glucose-6-phosphatase catalyzes the final step of both gluconeogenesis and glycogenolysis — the conversion of glucose-6-phosphate to free glucose. Its deficiency causes: 1. **Blocked glucose release** → severe hypoglycemia 2. **Glucose-6-phosphate accumulation** → shunted to: - Glycogen synthesis → hepatomegaly - Lactate pathway → lactic acidosis - Purine synthesis → hyperuricemia ```mermaid flowchart TD A[Glucose-6-Phosphate]:::outcome --> B{Glucose-6-phosphatase present?}:::decision B -->|Yes| C[Free Glucose released]:::action C --> D[Maintains blood glucose]:::outcome B -->|No| E[Glucose-6-P accumulates]:::urgent E --> F[Glycogen synthesis]:::action E --> G[Lactate formation]:::action E --> H[Purine synthesis]:::action F --> I[Hepatomegaly]:::outcome G --> J[Lactic acidosis]:::outcome H --> K[Hyperuricemia]:::outcome ``` ### Why This Is the Most Common Presentation **High-Yield:** Type I GSD accounts for ~25% of all glycogen storage disorders and is the most frequent cause of severe fasting hypoglycemia in infants. ### Metabolic Consequences | Metabolite | Mechanism | Clinical Effect | |-----------|-----------|------------------| | **Glucose** | Blocked release from G6P | Severe hypoglycemia (< 40 mg/dL) | | **Lactate** | G6P → glycolysis → lactate | Lactic acidosis (pH < 7.2) | | **Uric acid** | Increased purine degradation | Hyperuricemia, gout, renal stones | | **Triglycerides** | Increased fatty acid synthesis | Severe hypertriglyceridemia (> 1000 mg/dL) | | **Glycogen** | Accumulation in liver | Massive hepatomegaly (liver weight 2–3× normal) | ### Clinical Pearl The **3–4 hour fasting intolerance** is a cardinal feature that distinguishes Type I from other GSDs. Normal infants can tolerate 6–8 hours of fasting; Type I patients become symptomatic (seizures, loss of consciousness) within 3–4 hours. ### Management Principles - **Frequent feeding** (every 2–3 hours) or continuous nasogastric feeding - **Uncooked cornstarch** (provides sustained glucose release over 4–6 hours) - **Allopurinol** (prevents gout and renal complications) - **Avoid lactate-producing foods** (fructose, galactose) **Mnemonic:** **GLUT** (Glucose-6-phosphatase deficiency causes **G**lucose depletion, **L**actate accumulation, **U**ric acid elevation, **T**riglyceride elevation).