**Core Concept**
Galactosemia is a rare genetic disorder that affects the body's ability to metabolize the sugar galactose, which is found in milk and other dairy products. The classical type of galactosemia is caused by a deficiency in an enzyme involved in the breakdown of galactose.

**Why the Correct Answer is Right**
The classical type of galactosemia is caused by a deficiency in the enzyme galactose-1-phosphate uridyltransferase (GALT). This enzyme is responsible for converting galactose-1-phosphate into glucose-1-phosphate, which can then be used by the body for energy. Without functional GALT, galactose-1-phosphate accumulates in the body and can cause damage to the liver, kidneys, and brain. The deficiency of GALT leads to the inability to metabolize galactose, resulting in its accumulation and subsequent toxicity.

**Why Each Wrong Option is Incorrect**
**Option A:** Galactokinase is an enzyme involved in the metabolism of galactose, but its deficiency leads to a different type of galactosemia, known as galactokinase deficiency.
**Option B:** UDP-galactose 4'-epimerase is not directly involved in the classical type of galactosemia.
**Option C:** Glucose-6-phosphatase is an enzyme involved in gluconeogenesis, but its deficiency is associated with glycogen storage disease type I (von Gierke disease).

**Clinical Pearl / High-Yield Fact**
Galactosemia can be screened for at birth using a simple blood test, and early diagnosis and treatment can prevent serious complications.

**Correct Answer:** C.