A 3-day old baby feeds poorly, vomits on feeding. Urine Benedict's test is +ve. The reducing substance present in the urine is:

Galactosemia: Classic galactosemia is caused by almost total deficiency of galactose-1-phosphate uridyltransferase. Severely deficient neonates present with vomiting, jaundice, and hepatomegaly on initiation of lactose-containing feedings. Renal Fanconi syndrome, cataracts of the ocular lens, hepatic cirrhosis, and sepsis occur in untreated children. Delayed, apraxic speech and ovarian failure occur frequently even with treatment. Developmental delay, tremor, and ataxia occur less frequently. Urine gives positive Benedict's and Seliwanoffs tests. Galactose-1-phosphate is elevated in red blood cells. A galactose-free diet should be instituted as soon as the diagnosis is made. Ref: Thomas J.A., Van Hove J.L. (2012). Chapter 36. Inborn Errors of Metabolism. In W.W. Hay, Jr., M.J. Levin, R.R. Deterding, J.J. Ross, J.M. Sondheimer (Eds), CURRENT Diagnosis & Treatment: Pediatrics, 21e.