Commonest enzyme defect in galactossemia is:
**Core Concept**
Galactosemia is a genetic disorder characterized by the inability to metabolize the sugar galactose due to a deficiency in one of the enzymes involved in its metabolism. The most common form of galactosemia is due to a defect in the enzyme galactose-1-phosphate uridyltransferase.
**Why the Correct Answer is Right**
The correct answer is **Galactose-1-phosphate uridyltransferase (GALT)**. This enzyme catalyzes the conversion of galactose-1-phosphate to UDP-galactose, which is then used in the synthesis of glycogen and other glycoconjugates. Deficiency of GALT leads to the accumulation of galactose-1-phosphate, causing toxicity to the liver, kidney, and other organs.
**Why Each Wrong Option is Incorrect**
* **Option A:** Galactokinase is another enzyme involved in galactose metabolism, but it is less common than GALT deficiency.
* **Option B:** UDP-galactose-4'-epimerase is involved in the conversion of UDP-galactose to UDP-glucose, but it is not the most common enzyme defect in galactosemia.
* **Option C:** Galactose-6-phosphate dehydrogenase is not directly involved in the metabolism of galactose, and its deficiency is associated with a different condition, hemolytic anemia.
**Clinical Pearl / High-Yield Fact**
Galactosemia is often diagnosed in newborns through a screening test for elevated levels of galactose in the blood. If left untreated, galactosemia can lead to severe liver damage, cataracts, and intellectual disability.
**Correct Answer: C. Galactose-1-phosphate uridyltransferase (GALT)**