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?

Question

Accepted Answer

B. Accumulation of fructose-1,6-bisphosphate and shunting of glucose-6-phosphate into the pentose phosphate pathway and glycogen synthesis. ## 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.

Answer

A. Depletion of ATP and immediate loss of muscle contractility at rest

Answer

C. Inability to synthesize glucose-6-phosphate from glucose

Answer

D. Accumulation of pyruvate and lactate leading to severe metabolic acidosis

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

Metabolic Flow After PFK-1 Block

Key Metabolic Changes

Why Other Options Are Incorrect

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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