A 3-year-old boy from rural Maharashtra presents with severe hypoglycemia, hepatomegaly, and lactic acidosis following a 4-hour fast. Genetic testing reveals a deficiency in a glycolytic enzyme. Which enzyme deficiency is the most common cause of glycolytic enzyme deficiency presenting with this clinical picture?

## Pyruvate Kinase Deficiency: The Most Common Glycolytic Enzyme Deficiency ### Epidemiology & Frequency **Key Point:** Pyruvate kinase (PK) deficiency is the most common glycolytic enzyme deficiency, accounting for approximately 50% of all glycolytic enzyme defects. It is inherited in an autosomal recessive pattern. ### Clinical Presentation **High-Yield:** The classic triad includes: 1. **Fasting hypoglycemia** — often severe and symptomatic 2. **Hepatomegaly** — due to glycogen accumulation and impaired glucose output 3. **Lactic acidosis** — from shunting of glucose-6-phosphate into alternative pathways (pentose phosphate shunt, glycogen synthesis) ### Biochemical Mechanism **Clinical Pearl:** Pyruvate kinase catalyzes the final irreversible step of glycolysis: $$\text{Phosphoenolpyruvate (PEP)} + \text{ADP} \rightarrow \text{Pyruvate} + \text{ATP}$$ Deficiency leads to: - Accumulation of upstream glycolytic intermediates (glucose-6-phosphate, fructose-6-phosphate) - Shunting into glycogen synthesis → hepatomegaly - Shunting into lactate pathway → lactic acidosis - Impaired ATP generation → fasting hypoglycemia ### Comparison with Other Glycolytic Enzyme Deficiencies | Enzyme | Frequency | Key Features | Severity | | --- | --- | --- | --- | | **Pyruvate kinase** | Most common (50%) | Fasting hypoglycemia, hepatomegaly, lactic acidosis | Moderate to severe | | Phosphofructokinase | Rare | Myopathy, hemolysis, normal lactate | Variable | | Aldolase | Very rare | Hepatomegaly, hypoglycemia, neurological signs | Severe | | Hexokinase | Extremely rare | Fasting hypoglycemia, hyperglycemia paradox | Mild | **Mnemonic:** **PK FIRST** — Pyruvate Kinase is the First (most common) glycolytic deficiency ### Management Implications - Frequent small meals or continuous feeding to prevent hypoglycemia - Avoidance of prolonged fasting - Genetic counseling for autosomal recessive inheritance **Warning:** Do not confuse PK deficiency (glycolytic enzyme) with pyruvate dehydrogenase deficiency (mitochondrial enzyme) — different pathways, different presentations.