**Core Concept**
Galactosemia is a genetic disorder characterized by the inability to metabolize the sugar galactose. It occurs due to a deficiency in enzymes involved in the galactose metabolism pathway. The most common form of galactosemia is due to a deficiency in the enzyme galactose-1-phosphate uridyltransferase (GALT).

**Why the Correct Answer is Right**
Galactose-1-phosphate uridyltransferase (GALT) is a crucial enzyme in the Leloir pathway, which is responsible for converting galactose into glucose. In the absence of GALT, galactose accumulates in the body, leading to its toxic effects, such as liver dysfunction, cataracts, and neurological problems. The deficiency of GALT leads to the buildup of galactose-1-phosphate, which is a hallmark of classical galactosemia.

**Why Each Wrong Option is Incorrect**
* **Option A:** Galactokinase is another enzyme involved in the Leloir pathway, but its deficiency leads to a different form of galactosemia, characterized by cataract formation and mild symptoms.
* **Option B:** Galactose dehydrogenase is not a well-characterized enzyme in the galactose metabolism pathway.
* **Option D:** UDP-galactose 4'-epimerase is involved in the metabolism of galactose, but its deficiency is rare and does not lead to the classical symptoms of galactosemia.

**Clinical Pearl / High-Yield Fact**
Galactosemia is a classic example of a genetic disorder that can be diagnosed through newborn screening programs. Early diagnosis and treatment can prevent the development of severe symptoms and complications.

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