Question

All of the following enzyme deficiencies, except one, may cause lens opacities and mental retardation in a child. Which one is the exception?

• A. Galactokinase
• B. Galactose UDP 1 transferase
• C. Galactose 4 epimerase
• D. Lactase

Hint:

Only lactase sits outside the galactose metabolism pathway; the other three enzymes all handle galactose itself.

Answer 1

The Correct Option is D

Step 1: Understanding the Question.
The question lists four enzymes and asks which one, when deficient, does NOT cause lens opacities (cataracts) and mental retardation in a child. Three of the four enzymes work in galactose metabolism; only one has nothing to do with galactose handling.

Step 2: Trace the galactose pathway.
Dietary lactose is split into glucose and galactose. Galactose is then processed in three steps: galactokinase turns galactose into galactose-1-phosphate, galactose-1-phosphate uridyltransferase swaps it onto UDP-glucose to make UDP-galactose, and UDP-galactose-4-epimerase converts UDP-galactose back to UDP-glucose so it can enter normal glucose metabolism.

Step 3: Look at what happens when each galactose enzyme is missing.
A block at the transferase step (classic galactosemia) lets galactose-1-phosphate and galactose pile up. Some of that galactose is turned into galactitol by aldose reductase, and galactitol collects in the lens and draws in water, causing cataracts. The same buildup damages the liver and brain, causing mental retardation if the child stays on milk. A block at the epimerase step, in its severe generalized form, causes a similar pattern of cataracts and mental delay, because galactose and galactose-1-phosphate still build up. A block at the galactokinase step also raises blood galactose and lets galactitol form, so cataracts appear, and severe or late-treated cases carry some neurological risk too.

Step 4: Look at lactase.
Lactase is a different kind of enzyme. It sits on the brush border of the small intestine and splits lactose into glucose and galactose in the first place. If lactase is missing, lactose is never broken down and never absorbed, so galactose never even enters the blood in any extra amount. The child gets bloating, gas, and diarrhea from unabsorbed lactose reaching the colon, not lens opacities or mental retardation.

Step 5: Final Answer.
Since galactokinase, the transferase, and the epimerase all sit inside the galactose pathway and can each raise galactose or galactitol enough to affect the lens and brain, lactase is the odd one out.
\[ \boxed{\text{Lactase deficiency does not cause lens opacities and mental retardation}} \]