In galactosemia, which enzyme deficiency is most common?

• A. Galactose-1-phosphate uridyl transferase
• B. Galactokinase
• C. UDP galactose-4 epimerase
• D. Aldolase B

Answer: (A)

The main idea is identifying which enzyme defect is most often responsible for classic galactosemia. In the pathway that metabolizes galactose, galactose is first turned into galactose-1-phosphate by galactokinase, then galactose-1-phosphate is canalized into glucose-1-phosphate and UDP-galactose by galactose-1-phosphate uridyl transferase. When this latter enzyme is deficient, galactose-1-phosphate accumulates and galactose itself builds up, leading to the familiar neonatal illness with liver dysfunction, poor feeding, jaundice, and sometimes sepsis, as well as cataracts if untreated.

So the deficiency that most commonly causes galactosemia is the galactose-1-phosphate uridyl transferase deficiency. Other defects exist but are less common or produce different pictures: galactokinase deficiency mainly causes cataracts without the same systemic disease; UDP-galactose-4-epimerase deficiency is rare and variable; aldolase B is involved in fructose metabolism, not galactose.