A 3-year-old boy from rural Maharashtra presents with severe hypoglycemia (blood glucose 35 mg/dL) and hepatomegaly during a 6-hour fast. His parents report recurrent episodes of irritability, sweating, and tremor when meals are delayed. On examination, he has a doll-like face with full cheeks and a protuberant abdomen. Serum lactate is elevated at 8 mmol/L (normal <2), and uric acid is 9.2 mg/dL. A liver biopsy shows excessive glycogen accumulation. Which glycolytic enzyme deficiency is most likely responsible for this clinical picture?

Explanation

## Clinical Diagnosis: Glucose-6-Phosphatase Deficiency (GSD Type Ia) ### Key Clinical Features Pointing to G6Pase Deficiency **Key Point:** Glucose-6-phosphatase (G6Pase) catalyzes the final step of both glycogenolysis and gluconeogenesis, converting glucose-6-phosphate to free glucose in the endoplasmic reticulum. Its deficiency is the most severe glycogenosis. ### Pathophysiology 1. **Blocked final glucose release** → severe fasting hypoglycemia (within 3–4 hours) 2. **Accumulation of G6P** → shunted into: - Glycogen synthesis → massive hepatomegaly - Glycolysis → ↑ pyruvate → ↑ lactate (lactic acidosis) - Pentose phosphate pathway → ↑ NADPH → ↑ fatty acid synthesis → hypertriglyceridemia - Ribose-5-phosphate → ↑ purine synthesis → hyperuricemia (gout risk) ### Distinguishing Features of GSD Type Ia | Feature | GSD Ia (G6Pase) | GSD III (Cori) | GSD VI (PYGL) | |---------|-----------------|----------------|---------------| | **Enzyme** | Glucose-6-phosphatase | Debranching enzyme | Liver phosphorylase | | **Fasting hypoglycemia** | Severe (<40 mg/dL) | Mild–moderate | Mild | | **Hepatomegaly** | Massive (10–15 kg) | Moderate | Mild–moderate | | **Lactic acidosis** | **Marked** | Absent | Absent | | **Hyperuricemia** | **Yes (gout)** | No | No | | **Hypertriglyceridemia** | Severe | Mild | Mild | | **Muscle weakness** | No | **Yes (Type IIIb)** | No | | **Age of presentation** | Infancy (3–4 mo) | Infancy | Infancy | | **Prognosis** | Requires frequent feeds | Better (improves with age) | Good | ### Clinical Pearl **Clinical Pearl:** The triad of **severe fasting hypoglycemia + lactic acidosis + hyperuricemia** is pathognomonic for GSD Ia. The "doll-like face" (full cheeks, short stature) is a classic sign due to chronic malnutrition and hepatomegaly. ### High-Yield Biochemistry **High-Yield:** G6Pase is the **only enzyme that can release free glucose into the bloodstream**. Its deficiency is incompatible with normal fasting and requires frequent feeding (every 2–3 hours) or nocturnal nasogastric feeds with cornstarch. ### Management Implications - Frequent small meals or continuous nasogastric feeding - Uncooked cornstarch (provides slow glucose release) - Allopurinol for hyperuricemia - Ursodeoxycholic acid if cirrhosis develops ```mermaid flowchart TD A[Fasting state]:::outcome --> B[Glycogenolysis & Gluconeogenesis]:::action B --> C[G6P accumulation]:::urgent C --> D1[Glycogen synthesis]:::action C --> D2[Glycolysis → Pyruvate]:::action C --> D3[Pentose phosphate pathway]:::action C --> D4[Purine synthesis]:::action D1 --> E1[Massive hepatomegaly]:::outcome D2 --> E2[Lactic acidosis]:::outcome D3 --> E3[Hypertriglyceridemia]:::outcome D4 --> E4[Hyperuricemia + gout]:::outcome A --> F[No free glucose release]:::urgent F --> G[Severe hypoglycemia]:::urgent ``` [cite:Robbins 10e Ch 7]