A 28-year-old woman from Delhi presents with a 6-month history of progressive muscle weakness, myalgia, and dark urine after exercise. Laboratory investigations show elevated serum creatine kinase (CK: 8000 U/L), myoglobinuria, and a serum glucose of 48 mg/dL during a symptomatic episode of muscle pain and weakness. Muscle biopsy shows absent phosphofructokinase (PFK) activity. Which is the most appropriate immediate next step in management?

Explanation

## Clinical Context This patient has **glycogen storage disease type VII (GSD-VII, Tarui disease)**, caused by **phosphofructokinase (PFK) deficiency**—a critical enzyme in the glycolytic pathway. ### Pathophysiology **Key Point:** PFK catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate, a rate-limiting step in glycolysis. Its absence blocks ATP generation from glucose in muscle, leading to: - **Exercise intolerance** (no ATP for muscle contraction) - **Myoglobinuria** (muscle breakdown due to energy crisis) - **Hypoglycemia during exercise** (glucose cannot be metabolized) - **Elevated CK** (muscle necrosis) ## Why Option 1 (Exercise Avoidance + Frequent Carbohydrate Meals) Is Correct 1. **Prevents myoglobinuria:** Strenuous exercise triggers muscle breakdown in PFK deficiency. Strict avoidance prevents rhabdomyolysis and acute kidney injury. 2. **Provides alternative fuel:** Frequent small meals with carbohydrates supply glucose, which—though not metabolized via glycolysis—can be used via: - **Pentose phosphate pathway** (for NADPH) - **Direct ATP synthesis** from other substrates (amino acids, fatty acids via β-oxidation) 3. **Monitoring:** Urine myoglobin testing detects early muscle breakdown, prompting further intervention. 4. **Long-term:** Some patients benefit from **sucrose loading** before exercise (bypasses PFK via fructose metabolism). **Clinical Pearl:** Unlike GSD-I (glucose-6-phosphatase deficiency), which causes fasting hypoglycemia, GSD-VII causes **exercise-induced hypoglycemia** because muscle cannot use glucose for ATP production. **High-Yield:** The **"second wind" phenomenon** is pathognomonic for GSD-VII: patients feel better after 10 minutes of exercise because alternative fuels (fatty acids, amino acids) become available and bypass the PFK block. ## Why Each Distractor Is Wrong | Option | Reason | |--------|--------| | **High-dose corticosteroids** | Inflammation is secondary to energy crisis, not primary. Steroids do not restore PFK activity and may worsen catabolism. No evidence supports their use in GSD-VII. | | **Muscle MRI** | While MRI can assess muscle involvement, it does not change acute management. The diagnosis is already confirmed by enzyme assay (absent PFK). Imaging is not a management step. | | **Intravenous immunoglobulin** | IVIG is used for autoimmune myositis, not metabolic muscle disease. GSD-VII is not immune-mediated; IVIG will not restore PFK activity or prevent muscle breakdown. | ## Management Algorithm for GSD-VII ```mermaid flowchart TD A[Exercise-Induced Myalgia + Myoglobinuria]:::outcome --> B{PFK Deficiency Confirmed?}:::decision B -->|Yes| C[Strict Exercise Avoidance]:::action C --> D[Frequent Small Meals with Carbohydrates]:::action D --> E[Monitor Urine Myoglobin]:::action E --> F{Myoglobinuria Present?}:::decision F -->|Yes| G[Increase Meal Frequency & Hydration]:::action F -->|No| H[Maintain Current Regimen]:::outcome I[Consider Sucrose Loading Before Exertion]:::action D --> I ``` **Mnemonic:** **PFK-STOP** for GSD-VII management: - **P**FK deficiency (diagnosis) - **F**requent meals with carbohydrates - **K**eep exercise restricted - **S**econd wind phenomenon (reassure patient) - **T**est urine for myoglobin - **O**ptional: sucrose before exertion - **P**revent rhabdomyolysis & AKI [cite:Harrison 21e Ch 356; Robbins 10e Ch 7]