**Core Concept**
Galactosemia is a genetic disorder that affects the body's ability to metabolize the sugar galactose, which is found in milk and other dairy products. This condition is caused by a deficiency of an enzyme involved in the galactose metabolism pathway.

**Why the Correct Answer is Right**
Galactosemia is caused by a deficiency of the enzyme galactose-1-phosphate uridyltransferase (GALT). This enzyme plays a crucial role in the conversion of galactose-1-phosphate to UDP-galactose, which is then used to synthesize glycogen and other molecules. In the absence of GALT, galactose-1-phosphate accumulates in the body, leading to toxicity and various clinical manifestations.

**Why Each Wrong Option is Incorrect**
* **Option A:** is incorrect because it does not specify the enzyme deficient in galactosemia.
* **Option B:** is incorrect because it refers to a different enzyme involved in carbohydrate metabolism.
* **Option C:** is incorrect because it is not directly related to galactose metabolism.

**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 long-term complications, such as cataracts, liver dysfunction, and cognitive impairment.

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