A 3-year-old boy from rural Maharashtra presents with severe hypoglycemia (blood glucose 35 mg/dL) and hepatomegaly during fasting. Enzyme assay shows deficiency of glucose-6-phosphatase. His older sister, diagnosed at age 2, has normal fasting glucose but recurrent muscle pain and dark urine after exercise. Which feature best distinguishes the sister's condition from the boy's?

Explanation

## Distinguishing Glycogen Storage Disease Type I (GSD-I) from Type VII (Phosphofructokinase Deficiency) ### Clinical Presentation Comparison | Feature | GSD-I (G6Pase deficiency) | GSD-VII (PFK deficiency) | |---------|--------------------------|-------------------------| | **Fasting hypoglycemia** | Severe, <40 mg/dL | Normal or mild | | **Hepatomegaly** | Marked (massive) | Absent or mild | | **Lactate/Uric acid** | Elevated | Elevated only during exercise | | **Exercise intolerance** | Absent | Severe (myalgia, myoglobinuria) | | **Glycogen in liver** | Abundant | Abundant in muscle | | **Glycogen in muscle** | Normal | Markedly elevated | ### Key Distinguishing Feature **Key Point:** The sister's normal fasting glucose with elevated lactate and uric acid during exercise is pathognomonic for muscle-type glycogen storage disease (GSD-VII, phosphofructokinase deficiency), not hepatic GSD-I. **High-Yield:** - **GSD-I (Glucose-6-phosphatase deficiency):** Blocks final step of both gluconeogenesis and glycogenolysis → severe fasting hypoglycemia, massive hepatomegaly, lactic acidosis at rest. - **GSD-VII (PFK deficiency):** Blocks glycolysis in muscle → normal fasting glucose (liver glycogenolysis intact), exercise-induced myalgia, myoglobinuria, and lactate accumulation only with exertion. **Clinical Pearl:** The "second wind" phenomenon in GSD-VII is classic: initial exercise intolerance improves after 10 minutes as alternative fuel sources (fatty acids, amino acids) become available. **Mnemonic:** **LIVER vs MUSCLE** - **L**iver GSD (Type I): **L**actate elevated at **REST**, **L**arge liver - **M**uscle GSD (Type VII): **M**yalgia with **M**ovement, **M**uscle glycogen accumulation ### Why This Discriminates The boy has hepatic enzyme deficiency (glucose-6-phosphatase) → cannot complete either gluconeogenesis or glycogenolysis → severe fasting hypoglycemia and hepatomegaly. The sister has muscle enzyme deficiency (phosphofructokinase) → liver glycogenolysis intact (normal fasting glucose) → but muscle cannot metabolize glucose-6-phosphate → accumulates glycogen in muscle and causes exercise-induced symptoms with lactate/uric acid elevation. [cite:Robbins 10e Ch 7] [cite:KD Tripathi 8e Ch 12]