A 3-year-old boy from rural Maharashtra presents with recurrent episodes of severe hypoglycemia, lactic acidosis (pH 7.1, lactate 8 mmol/L), and hepatomegaly. His parents report that symptoms worsen during fasting and improve with frequent meals. Serum pyruvate is elevated at 4.2 mg/dL (normal <2). Blood glucose during a fasting challenge drops to 32 mg/dL within 4 hours. Liver biopsy shows glycogen accumulation. Which enzyme deficiency best explains this clinical presentation?

Clinical Diagnosis: Pyruvate Carboxylase Deficiency

Pathophysiology

Pyruvate carboxylase catalyzes the first committed step of gluconeogenesis:

Deficiency of this enzyme impairs gluconeogenesis while glycolysis continues unchecked, leading to:

Key Point:

Pyruvate carboxylase is the rate-limiting enzyme of gluconeogenesis. Its deficiency is one of the most severe forms of hypoglycemia in infancy, presenting with fasting intolerance and lactic acidosis.

Clinical Pearl:

The hallmark triad of pyruvate carboxylase deficiency:

High-Yield:

Pyruvate carboxylase deficiency is autosomal recessive and presents in the neonatal or early infancy period (usually by age 3 months to 3 years). Unlike other glycogen storage diseases, hepatomegaly is present but not the dominant feature.

Differential Diagnosis Table

Tip:

Fasting challenge test is diagnostic: blood glucose drops rapidly (within 4 hours) with simultaneous rise in lactate and pyruvate — this is pathognomonic for pyruvate carboxylase deficiency.