The enzyme defect in galactosemia is
**Core Concept**
Galactosemia is a genetic disorder characterized by the inability to metabolize galactose, a sugar found in milk and dairy products. The disorder is caused by a deficiency in one of the enzymes involved in the Leloir pathway, which is responsible for converting galactose into glucose.
**Why the Correct Answer is Right**
The enzyme defect in galactosemia is due to a deficiency in galactose-1-phosphate uridyltransferase (GALT), an enzyme that catalyzes the second step of the Leloir pathway. This enzyme is critical for the conversion of galactose-1-phosphate to UDP-galactose, which is then converted to glucose-1-phosphate. Without GALT, galactose accumulates in the body and can cause damage to the liver, kidneys, and brain. The deficiency of GALT is responsible for the classic form of galactosemia.
**Why Each Wrong Option is Incorrect**
* **Option A:** Galactokinase (GALK) is an enzyme that catalyzes the first step of the Leloir pathway, but its deficiency leads to a different form of galactosemia, characterized by cataracts and hearing loss.
* **Option B:** UDP-galactose-4'-epimerase (GALE) is an enzyme that catalyzes the conversion of UDP-galactose to UDP-glucose, but its deficiency is not associated with galactosemia.
* **Option D:** Galactose mutarotase is an enzyme that catalyzes the conversion of D-galactose to beta-D-galactose, but it is not involved in the Leloir pathway and is not associated with galactosemia.
**Clinical Pearl / High-Yield Fact**
Galactosemia is often diagnosed in newborns through a screening test that detects elevated levels of galactose-1-phosphate in the blood. Untreated galactosemia can lead to liver failure, sepsis, and death, highlighting the importance of early diagnosis and treatment.
**Correct Answer:** C.