Question:hard
All of the following enzyme deficiencies, except one, may cause lens opacities and mental retardation in a child. Which one is the exception?
All of the following enzyme deficiencies, except one, may cause lens opacities and mental retardation in a child. Which one is the exception?
Show Hint
Only lactase sits outside the galactose metabolism pathway; the other three enzymes all handle galactose itself.
Updated On: Jul 8, 2026
- Galactokinase
- Galactose UDP 1 transferase
- Galactose 4 epimerase
- Lactase
Show Solution
The Correct Option is D
Solution and Explanation
Step 1: Sort the enzymes by pathway.
Three of the four enzymes, galactokinase, the galactose-1-phosphate uridyltransferase, and galactose-4-epimerase, all work one after another to turn galactose into a form the body can use as glucose. Lactase stands apart; it lives in the gut wall and only cuts lactose into glucose and galactose before absorption.
Step 2: Follow what builds up in each block.
When any of the three galactose-handling enzymes is missing, galactose and galactose-1-phosphate cannot move forward properly and build up in the blood and tissues. Part of that extra galactose gets reduced to galactitol inside the lens, and galactitol pulls water in by osmosis, swelling the lens fibers and clouding them into a cataract. Ongoing buildup also injures the brain, giving mental retardation if untreated.
Step 3: Check lactase separately.
If lactase is missing, lactose simply cannot be split at all, so galactose never gets absorbed into the bloodstream in the first place. There is nothing to build up in the lens or brain from a lactase block. Symptoms instead stay confined to the gut, as cramping, bloating, and loose stools after drinking milk.
Step 4: State the answer.
Because lactase acts before galactose absorption and the other three act after it, only a lactase deficiency spares the child from lens opacities and mental retardation.
$\boxed{\text{Lactase}}$
Three of the four enzymes, galactokinase, the galactose-1-phosphate uridyltransferase, and galactose-4-epimerase, all work one after another to turn galactose into a form the body can use as glucose. Lactase stands apart; it lives in the gut wall and only cuts lactose into glucose and galactose before absorption.
Step 2: Follow what builds up in each block.
When any of the three galactose-handling enzymes is missing, galactose and galactose-1-phosphate cannot move forward properly and build up in the blood and tissues. Part of that extra galactose gets reduced to galactitol inside the lens, and galactitol pulls water in by osmosis, swelling the lens fibers and clouding them into a cataract. Ongoing buildup also injures the brain, giving mental retardation if untreated.
Step 3: Check lactase separately.
If lactase is missing, lactose simply cannot be split at all, so galactose never gets absorbed into the bloodstream in the first place. There is nothing to build up in the lens or brain from a lactase block. Symptoms instead stay confined to the gut, as cramping, bloating, and loose stools after drinking milk.
Step 4: State the answer.
Because lactase acts before galactose absorption and the other three act after it, only a lactase deficiency spares the child from lens opacities and mental retardation.
$\boxed{\text{Lactase}}$
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