A 3 month old baby refuses breast milk since the 2nd day of birth, but accepts glucose-water, develops vomiting and severe jaundice by the 5th day. Benedict's test was positive for urine and blood glucose was low. The most likely cause is due to the deficiency of:

The newborn infant receives high amounts of lactose, which consists of equal parts of glucose and galactose. Without the galactose-1-phosphate uridyl transferase enzyme, the infant is unable to metabolize galactose-1- phosphate, the accumulation of which results in injury to kidney, liver and brain. Clinical features: Classic galactosemia is a serious disease with onset of symptoms typically by the 2nd half of the 1st week of life. With jaundice, vomiting, seizures, lethargy, irritability, feeding difficulties, poor weight gain or failure to regain birth weight. Hepatomegaly Oil drop cataracts, Hepatic failure. Mental retardation. Patients with galactosemia are at increased risk for Escherichia coli neonatal sepsis. Diagnosis: Demonstrating a reducing substance in several urine specimens collected while the patient is receiving human milk, cow’s milk or any other formula containing lactose. Benedict’s test positive. ​Reference- Harper’s illustrated biochemistry. 30th edition page -203