A 35-year-old woman with a history of recurrent muscle cramps and myoglobinuria during exercise is found to have a deficiency of phosphofructokinase-1 (PFK-1). Which is the most common metabolic consequence of this enzyme deficiency?

Explanation

## PFK-1 Deficiency and Metabolic Consequences ### Pathophysiology of PFK-1 Deficiency PFK-1 deficiency (also called Tarui disease or glycogen storage disease type VII) blocks the committed step of glycolysis. The most common consequence is the **accumulation of upstream substrates and shunting into alternative pathways**. ### Metabolic Flow After PFK-1 Block ```mermaid flowchart TD A["Glucose"]:::outcome --> B["Glucose-6-phosphate"]:::outcome B --> C{"PFK-1 blocked"}:::decision C -->|"Accumulates"| D["Fructose-1,6-bisphosphate"]:::outcome C -->|"Shunted away"| E["Pentose Phosphate Pathway"]:::action C -->|"Shunted away"| F["Glycogen Synthesis"]:::action D -->|"Backup"| B B --> G["Glycogen accumulation"]:::urgent E --> H["NADPH production"]:::outcome F --> G ``` ### Key Metabolic Changes **Key Point:** In PFK-1 deficiency, glucose-6-phosphate accumulates because it cannot proceed through glycolysis. It is shunted into: - **Pentose phosphate pathway** → NADPH production (for biosynthesis and antioxidant defense) - **Glycogen synthesis** → glycogen accumulation in muscle ### Why Other Options Are Incorrect | Consequence | Why Not Most Common | |-------------|---------------------| | ATP depletion at rest | Patients are asymptomatic at rest; symptoms occur only with exercise when glycolysis is required | | Pyruvate/lactate accumulation | Glycolysis is BLOCKED before pyruvate formation; pyruvate production is actually DECREASED | | Inability to make G6P | Hexokinase is intact; G6P is produced normally but cannot proceed through glycolysis | **High-Yield:** PFK-1 deficiency causes **"second wind" phenomenon** — patients experience severe exercise intolerance initially, but symptoms may improve with continued exercise as alternative fuels (fatty acids, amino acids) become available. This is pathognomonic for PFK-1 deficiency. **Clinical Pearl:** Unlike other glycogen storage diseases (e.g., GSD Type I with hepatomegaly), PFK-1 deficiency primarily affects **skeletal muscle**, causing exercise intolerance and myoglobinuria rather than hepatic manifestations. **Mnemonic:** **PFK Block = Pentose Phosphate & Glycogen Build-up** — upstream substrates accumulate and are diverted away from the blocked glycolytic pathway.