A 28-year-old woman from Delhi presents with acute onset of severe muscle pain, dark urine, and myoglobinuria following intense exercise at a gym. Serum creatine kinase is 8,500 U/L (normal <200). Blood glucose is 78 mg/dL, lactate is 1.2 mmol/L (normal), and venous pH is 7.38. Muscle biopsy shows subsarcolemmal glycogen accumulation and absent acid maltase activity. Which glycolytic/glycogenolytic enzyme deficiency would explain the normal lactate despite severe muscle breakdown?

Explanation

## Clinical Diagnosis: Phosphofructokinase-1 (PFK-1) Deficiency (Tarui Disease, GSD VII) ### Pathophysiology PFK-1 catalyzes the committed step of glycolysis: $$\text{Fructose-6-phosphate} + \text{ATP} \rightarrow \text{Fructose-1,6-bisphosphate} + \text{ADP}$$ Deficiency of PFK-1 causes: 1. **Glycolytic block** — glucose-6-phosphate and upstream intermediates accumulate 2. **Shunting to glycogen synthesis** — G6P is diverted to UDP-glucose → glycogen 3. **Impaired ATP generation from glycolysis** — muscle cannot generate energy during exercise 4. **Normal or LOW lactate** — because glycolysis is blocked BEFORE pyruvate formation 5. **Rhabdomyolysis** — energy crisis in muscle leads to myoglobinuria and elevated CK ### Key Point: **PFK-1 deficiency is the ONLY glycolytic enzyme deficiency that causes normal or LOW lactate during exercise-induced rhabdomyolysis.** This is the critical distinguishing feature from other causes of muscle breakdown. ### High-Yield: Tarui disease (GSD VII) presents with: - **Exercise intolerance** and **muscle cramps** (second wind phenomenon: symptoms improve after 10 minutes as alternative fuels activate) - **Myoglobinuria** and rhabdomyolysis after intense exercise - **Hemolytic anemia** (PFK-1 deficiency in RBCs causes osmotic fragility) - **Normal or low lactate** (glycolytic block) - **Subsarcolemmal glycogen accumulation** on biopsy ### Clinical Pearl: **"Second wind" phenomenon** is pathognomonic for PFK-1 deficiency: symptoms worsen initially during exercise, then improve after ~10 minutes as alternative fuels (fatty acids, ketones) become available. This distinguishes it from phosphorylase deficiency (which shows persistent symptoms). ### Comparison Table: Glycolytic Enzyme Deficiencies | Enzyme | Block Site | Lactate | CK | Hemolysis | Glycogen | Key Feature | | --- | --- | --- | --- | --- | --- | --- | | **PFK-1** | G6P → F1,6BP | ↓ Normal | ↑↑↑ | Yes | Subsarcolemmal | Second wind; normal lactate | | Phosphorylase | Glycogen → G1P | Normal | ↑ | No | Massive | McArdle disease; high lactate | | Hexokinase | Glucose → G6P | ↓ | ↑ | No | Minimal | Severe hypoglycemia | | Acid maltase | Lysosomal glycogen | Normal | ↑↑ | No | Massive (lysosomal) | Infantile: cardiomyopathy | ### Tip: **The lactate paradox** — in most causes of rhabdomyolysis, lactate is elevated (anaerobic metabolism). In PFK-1 deficiency, lactate is normal or LOW because the glycolytic block prevents pyruvate formation. This is the key discriminator on the exam.