A 28-year-old woman from Delhi presents with a 6-month history of progressive muscle weakness, myalgia, and dark-colored urine after intense exercise. She reports that her symptoms worsen with strenuous activity and improve with rest. Serum creatine kinase is 3200 U/L, and urine myoglobin is positive. A muscle biopsy shows glycogen accumulation. During ischemic forearm exercise test, venous lactate fails to rise (remains <1 mmol/L) while ammonia increases markedly. Which enzyme deficiency is most likely responsible for her symptoms?

Explanation

## Clinical Diagnosis: Glycogen Storage Disease Type VII (Tarui Disease) ### Pathophysiology of PFK-1 Deficiency **Key Point:** Phosphofructokinase-1 (PFK-1) catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate — the first committed step of glycolysis and a major regulatory enzyme. Its deficiency blocks glycolysis in skeletal muscle and red blood cells. ### Why This Presentation Fits PFK-1 Deficiency 1. **Exercise-induced myalgia and muscle weakness** — glycolysis is blocked, so muscles cannot generate ATP from glucose; they rely on creatine phosphate and oxidative phosphorylation, which are insufficient during intense exercise. 2. **Myoglobinuria (dark urine)** — muscle necrosis from energy depletion during exercise leads to rhabdomyolysis and myoglobin release. 3. **Elevated CK** — marker of muscle damage from ATP depletion. 4. **Absent lactate rise during ischemic exercise test** — lactate is the end product of glycolysis; if glycolysis is blocked at PFK-1, lactate cannot be generated despite muscle activity. 5. **Elevated ammonia** — muscles break down adenine nucleotides (ATP → ADP → AMP → adenosine → ammonia) to generate energy when glycolysis fails. This is the **second wind phenomenon** — after initial fatigue, patients may improve slightly as oxidative phosphorylation takes over. 6. **Glycogen accumulation** — glucose-6-phosphate and fructose-6-phosphate accumulate and are converted back to glycogen. 7. **Hemolytic anemia** — RBCs also depend on glycolysis for ATP; PFK-1 deficiency causes chronic hemolysis (not mentioned in this vignette but a key feature of GSD-VII). ### Ischemic Forearm Exercise Test: The Diagnostic Gold Standard | Parameter | Normal | PFK-1 Deficiency | Other Glycolytic Defects | |-----------|--------|------------------|-------------------------| | **Venous lactate rise** | 5–10× baseline | Absent or minimal (<1 mmol/L) | Absent | | **Venous ammonia rise** | 2–3× baseline | Markedly elevated (>5× baseline) | Normal or mild rise | | **Muscle pain/cramps** | None | Severe | Absent or mild | | **Myoglobinuria** | No | Yes (with intense exercise) | No | **High-Yield:** The **absent lactate rise + markedly elevated ammonia** during ischemic exercise is pathognomonic for PFK-1 deficiency and distinguishes it from other glycolytic enzyme defects. ### Why Other Enzymes Don't Fit - **Glucose-6-phosphatase (GSD-I):** Causes fasting hypoglycemia and lactic acidosis at rest, not exercise-induced myalgia. Lactate is elevated, not absent. - **Lactate dehydrogenase (LDH):** Deficiency is extremely rare; it does not cause glycogen accumulation or exercise intolerance. Lactate would accumulate (not absent). - **Aldolase:** Deficiency causes hepatomegaly and hemolytic anemia, not exercise-induced rhabdomyolysis or absent lactate. ### Management 1. **Avoid strenuous exercise** — the primary trigger for symptoms. 2. **Frequent small meals** with complex carbohydrates to maintain blood glucose. 3. **Sucrose loading** before exercise — fructose can bypass the PFK-1 block and provide some ATP. 4. **Genetic counseling** — autosomal recessive inheritance. **Clinical Pearl:** Patients with GSD-VII often report a "second wind" phenomenon — initial severe fatigue and myalgia improve after 10–15 minutes of rest, as oxidative phosphorylation compensates for the glycolytic block. This is a helpful diagnostic clue. [cite:Robbins 10e Ch 5; Harrison 21e Ch 354; KD Tripathi 8e Ch 32]