Deficiency of Galactose-1-phosphate uridyltransferase (GALT) is the primary cause of classic galactosemia. This enzyme is crucial for the proper metabolism of galactose, converting galactose-1-phosphate into UDP-galactose. Its deficiency leads to the accumulation of toxic substances such as galactose-1-phosphate in tissues, causing liver damage, jaundice, cataracts, and intellectual disability if untreated. Early diagnosis and dietary restriction of galactose are essential to prevent these severe complications.

Reference: Principles of Biochemistry, Nelson & Cox, 7th Edition.