A 3-year-old boy from rural Maharashtra presents with severe hypoglycemia (blood glucose 35 mg/dL) and hepatomegaly during a routine febrile illness. His parents report that he becomes irritable and sweaty whenever meals are delayed by more than 2 hours. On examination, he has a doll-like face with full cheeks and a protuberant abdomen. Serum lactate is markedly elevated at 8 mmol/L (normal <2), and arterial blood gas shows metabolic acidosis. Liver biopsy reveals excessive glycogen accumulation. Which enzyme deficiency is most likely responsible for this clinical presentation?

Explanation

## Clinical Diagnosis: Glucose-6-Phosphatase Deficiency (Glycogen Storage Disease Type I) ### Key Pathophysiology **Key Point:** Glucose-6-phosphatase catalyzes the final step of both gluconeogenesis and glycogenolysis — the conversion of glucose-6-phosphate to free glucose. Without this enzyme, glucose cannot be released into the bloodstream, leading to severe fasting hypoglycemia and lactic acidosis. ### Why This Patient Has These Findings | Feature | Mechanism | Glucose-6-Phosphatase Deficiency | |---------|-----------|----------------------------------| | **Severe fasting hypoglycemia** | Cannot release glucose from liver | Symptomatic within 2–3 hours of fasting | | **Hepatomegaly** | Glucose-6-P accumulates → glycogen synthesis | Massive hepatomegaly (up to 2–3× normal size) | | **Lactic acidosis** | Glucose-6-P shunted to glycolysis → pyruvate → lactate | Lactate >5 mmol/L is characteristic | | **Doll-like facies** | Growth retardation from chronic hypoglycemia | Delayed growth, "cherubic" appearance | | **Excessive liver glycogen** | Glycogen synthesis unopposed by glucose release | Histology shows glycogen-laden hepatocytes | ### High-Yield Clinical Features **High-Yield:** GSD Type I (Von Gierke disease) is the most severe glycogenosis. The classic triad is: 1. **Severe fasting hypoglycemia** (blood glucose <40 mg/dL within 3–4 hours of fasting) 2. **Massive hepatomegaly** (liver can weigh >2 kg) 3. **Lactic acidosis** (lactate often >5 mmol/L) **Clinical Pearl:** The "doll-like face" with full cheeks and a protuberant abdomen is pathognomonic for GSD Type I. The full cheeks are due to fat deposition, and the protuberant abdomen is from massive hepatomegaly. ### Biochemical Mechanism ```mermaid flowchart TD A[Glycogen breakdown & Gluconeogenesis]:::action --> B[Glucose-6-Phosphate accumulates]:::outcome B --> C{Glucose-6-Phosphatase present?}:::decision C -->|Normal| D[Free glucose released]:::action C -->|Deficient| E[Glucose-6-P shunted to glycolysis]:::urgent D --> F[Normal blood glucose]:::outcome E --> G[Pyruvate → Lactate]:::action E --> H[Glucose-6-P → Glycogen synthesis]:::action G --> I[Lactic acidosis]:::urgent H --> J[Hepatomegaly]:::urgent ``` ### Diagnostic Confirmation **Key Point:** Diagnosis is made by: - Severe fasting hypoglycemia with elevated lactate (>5 mmol/L) - Massive hepatomegaly with glycogen accumulation on biopsy - Genetic testing: mutations in *G6PC* gene - Enzyme assay: absent or severely reduced glucose-6-phosphatase activity in liver ### Management Principles 1. **Frequent feeding** (every 2–3 hours) with complex carbohydrates 2. **Cornstarch** (uncooked starch) — provides slow glucose release 3. **Nasogastric feeding** at night to prevent nocturnal hypoglycemia 4. **Avoid fasting** — even brief fasting causes severe hypoglycemia [cite:Robbins 10e Ch 7]