A 3-year-old boy from rural Maharashtra presents with recurrent episodes of severe hypoglycemia, muscle pain, and dark urine after prolonged fasting or exercise. Physical examination reveals hepatomegaly and mild muscle weakness. Serum lactate is markedly elevated (8 mmol/L, normal <2), and blood glucose drops to 35 mg/dL after a 6-hour fast. Muscle biopsy shows accumulation of glycogen with normal structure. Which enzyme deficiency best explains this clinical presentation?

Explanation

## Clinical Diagnosis: Glycogen Storage Disease Type VII (GSD VII) ### Key Biochemical Defect **Key Point:** Phosphofructokinase (PFK) deficiency in muscle (the muscle isoform, encoded by PFKM gene) causes GSD VII, also called Tarui disease. PFK catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate — a critical regulatory step in glycolysis. ### Why This Patient Fits 1. **Severe hypoglycemia on fasting/exercise** — muscle cannot mobilize glucose via glycolysis; glycogen accumulates but cannot be efficiently catabolized. 2. **Elevated serum lactate** — glycolysis is blocked at PFK, but glucose that does enter glycolysis shunts toward lactate via alternative pathways (Cori cycle overflow). 3. **Muscle pain and weakness** — energy crisis in muscle during exertion; ATP depletion triggers myalgia and fatigue. 4. **Dark urine** — myoglobinuria from muscle breakdown (rhabdomyolysis during exertion). 5. **Normal glycogen structure** — the defect is in *utilization*, not synthesis. ### Glycolysis Checkpoint ```mermaid flowchart TD A[Glucose] -->|Hexokinase| B[Glucose-6-P] B -->|Phosphoglucose isomerase| C[Fructose-6-P] C -->|PFK| D[Fructose-1,6-BP]:::action D -->|Aldolase| E[DHAP + G3P] E -->|G3PDH| F[1,3-BPG] F -->|PGK| G[3-PG] G -->|PGM| H[2-PG] H -->|Enolase| I[PEP] I -->|PK| J[Pyruvate] J -->|LDH| K[Lactate] C -->|Shunt| L[G6P → Pentose Pathway]:::outcome style D fill:#ffcccc classDef action fill:#10b981 classDef outcome fill:#3b82f6 ``` **High-Yield:** PFK is the rate-limiting enzyme of glycolysis and the primary control point. Its deficiency causes the most severe glycolytic block in GSD VII. ### Clinical Pearl **Clinical Pearl:** Patients with PFK deficiency often report a "second wind" phenomenon — initial exercise triggers pain/cramping, but if they rest briefly and resume activity, symptoms improve. This reflects shunting to alternative ATP sources (fatty acids, amino acids). ### Diagnostic Confirmation - **Ischemic forearm exercise test** — normal rise in venous lactate is absent or blunted; ammonia rises excessively (purine degradation compensates for ATP deficit). - **Muscle biopsy** — glycogen accumulation, normal structure, PFK activity <5% of normal. - **Genetic testing** — PFKM gene mutation. [cite:KD Tripathi 8e Ch 8]