A 6-month-old girl from Delhi is brought to the pediatric emergency with a history of vomiting, abdominal pain, and hypoglycemia (blood glucose 38 mg/dL) that developed 2 hours after her mother fed her honey and fruit juice. On examination, she is irritable with hepatomegaly. Serum fructose is markedly elevated. Liver function tests show elevated transaminases (AST 320 U/L, ALT 280 U/L) and hyperbilirubinemia. Which enzyme deficiency best explains this acute presentation?