A 3-year-old child presents with severe hypoglycemia (blood glucose 35 mg/dL) and hepatomegaly. The child is asymptomatic during fasting but develops symptoms 2–3 hours after meals. Serum lactate is elevated at 8 mmol/L (normal <2), and uric acid is 9.2 mg/dL. Which investigation is most appropriate to confirm the suspected diagnosis of glycogen storage disease type I (Von Gierke disease)?

Explanation

## Diagnostic Approach to GSD Type I ### Clinical Context The presentation—fasting hypoglycemia, hepatomegaly, elevated lactate, and hyperuricemia—is pathognomonic for glycogen storage disease type I (GSD-I or Von Gierke disease), which results from deficiency of glucose-6-phosphatase (G6Pase), the final enzyme in both gluconeogenesis and glycogenolysis. ### Why Liver Biopsy with Enzyme Assay is the Gold Standard **Key Point:** Liver biopsy with direct measurement of glucose-6-phosphatase activity is the definitive diagnostic test for GSD-I because: 1. It directly demonstrates the enzyme deficiency in the organ most affected (liver) 2. It allows quantification of residual enzyme activity (typically <10% of normal in GSD-I) 3. It permits histological confirmation of glycogen accumulation 4. It is the reference standard against which all other tests are validated ### Biochemical Basis Glucose-6-phosphatase catalyzes the final step of gluconeogenesis: $$G6P \xrightarrow{G6Pase} Glucose + Pi$$ Without this enzyme: - Glucose cannot be released from liver → severe fasting hypoglycemia - G6P accumulates → shunted to glycogen synthesis (hepatomegaly) and lactate production (lactic acidosis) - Increased purine degradation from ATP consumption → hyperuricemia **High-Yield:** The elevated lactate and hyperuricemia are secondary consequences of G6P accumulation, not primary defects. They confirm the diagnosis but do not identify the specific enzyme deficiency. ### Timing and Clinical Utility Enzyme assay on liver tissue is performed on fresh or snap-frozen samples and provides: - Definitive diagnosis within hours to days - Quantitative data on enzyme activity - Substrate accumulation patterns (glycogen, lipid) **Clinical Pearl:** In resource-limited settings or when biopsy is contraindicated, genetic testing for G6PC gene mutations is an acceptable alternative, but enzyme assay remains the gold standard for confirmation and prognosis assessment. [cite:Robbins 10e Ch 7]