Diagnostic test for galactosemia measures activity of which enzyme in erythrocytes?

• A. Galactose-1-phosphate uridyl transferase
• B. Lactase
• C. Galactose mutarotase
• D. Hexokinase

Answer: (A)

Galactose-1-phosphate uridyl transferase activity in erythrocytes is measured because it is the first committed step in the galactose metabolism pathway used to diagnose galactosemia. In the normal Leloir pathway, this enzyme transfers a uridylyl group from UDP-glucose to galactose-1-phosphate, producing glucose-1-phosphate and UDP-galactose. If the enzyme is deficient, galactose-1-phosphate and other toxic metabolites accumulate, causing the clinical manifestations of galactosemia. Assessing this specific enzyme’s activity in red blood cells provides a direct indication of the dysfunctional step. The other enzymes listed are not involved in this diagnostic context: lactase acts on dietary lactose in the gut, galactose mutarotase is a minor isomerase not used for diagnostic testing, and hexokinase phosphorylates hexoses including glucose and galactose but is not used to diagnose galactosemia.