A 3-year-old boy from rural Maharashtra presents with severe hypoglycemia (blood glucose 35 mg/dL), hepatomegaly, and lactic acidosis (pH 7.2, lactate 8 mmol/L) following a 6-hour fast. Clinical suspicion is high for a glycolytic enzyme deficiency. Which investigation is most specific for confirming phosphofructokinase (PFK) deficiency?

Explanation

## Diagnosis of Phosphofructokinase Deficiency ### Why Erythrocyte PFK Enzyme Activity Assay is Correct **Key Point:** PFK deficiency (Tarui disease) is an autosomal recessive glycolytic disorder. The enzyme phosphofructokinase catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate — a rate-limiting step in glycolysis. **High-Yield:** PFK is expressed in multiple tissues (muscle, RBCs, liver, brain). Erythrocytes are easily accessible and reliably reflect enzymatic deficiency. Direct enzyme activity measurement is the gold standard for confirming the diagnosis. **Clinical Pearl:** Patients with PFK deficiency present with: - Severe hypoglycemia and lactic acidosis (substrate accumulation proximal to the block) - Hepatomegaly (glycogen accumulation) - Myopathy and hemolytic anemia (RBC involvement) - Symptoms triggered by fasting or intense exercise ### Why Other Investigations Are Less Specific | Investigation | Limitation | |---|---| | Serum pyruvate/lactate | Non-specific; elevated in many glycolytic/mitochondrial disorders | | Liver biopsy | Shows glycogen accumulation but does NOT identify the specific enzyme defect | | ALDOLASE gene sequencing | Aldolase deficiency causes a different glycolytic disorder; wrong gene target | **Mnemonic:** SPECIFIC ENZYME → SPECIFIC TISSUE → SPECIFIC ASSAY. RBC PFK activity is the definitive test. [cite:KD Tripathi 8e Ch 8]