A 3-year-old boy from rural Maharashtra presents with recurrent episodes of severe hypoglycemia, lactic acidosis, and hepatomegaly. His parents report that symptoms worsen during fasting and improve after meals. Laboratory investigations show: blood glucose 35 mg/dL (fasting), lactate 8 mmol/L (normal <2), pyruvate 2.5 mmol/L (normal <0.1), and normal liver function tests. Genetic testing confirms homozygous loss-of-function mutation in the LDHA gene encoding lactate dehydrogenase A. Which glycolytic enzyme is MOST likely to be functionally impaired as a consequence of this mutation, leading to the observed metabolic derangement?