**Core Concept**
Galactosemia is a genetic disorder characterized by the inability to properly break down galactose, a sugar found in milk and other dairy products, due to a deficiency in one of the enzymes involved in its metabolism. The most common form of galactosemia is caused by a deficiency in the enzyme responsible for converting galactose-1-phosphate to glucose-1-phosphate.

**Why the Correct Answer is Right**
The correct enzyme deficiency responsible for galactosemia is a deficiency in the enzyme galactose-1-phosphate uridyltransferase (GALT). This enzyme catalyzes the transfer of a uridine diphosphate (UDP) group from UDP-glucose to galactose-1-phosphate, forming UDP-galactose and glucose-1-phosphate. In the absence of GALT, galactose-1-phosphate accumulates in the body, leading to the clinical manifestations of galactosemia. The GALT enzyme is a tetrameric enzyme, composed of four subunits, and mutations in the GALT gene can lead to a reduction or complete loss of enzyme activity.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not a valid enzyme related to galactose metabolism.
* **Option B:** Glucose-6-phosphatase deficiency is responsible for von Gierke's disease, a different glycogen storage disorder.
* **Option C:** This option is not a correct enzyme related to galactose metabolism.

**Clinical Pearl / High-Yield Fact**
Galactosemia can be screened for at birth using a simple urine test that detects the presence of galactose-1-phosphate in the urine. Early diagnosis and treatment can prevent the development of long-term complications, such as cataracts, liver dysfunction, and cognitive impairment.

**Correct Answer:** C. Galactose-1-phosphate uridyltransferase (GALT).