**Core Concept**
Classic Galactosemia is an inherited metabolic disorder caused by the deficiency of an enzyme essential for the proper metabolism of galactose, a sugar found in milk and dairy products. This condition leads to the accumulation of galactose-1-phosphate in the body, causing various systemic complications. The affected enzyme catalyzes a critical step in the Leloir pathway, which is responsible for converting galactose into glucose for further utilization.

**Why the Correct Answer is Right**
Galactose-1-Phosphate-Uridyl-Transferase (GALT) is the enzyme responsible for converting galactose-1-phosphate into UDP-galactose. This step is crucial in the Leloir pathway, as it allows galactose to be further processed and converted into glucose. A deficiency in GALT leads to the accumulation of galactose-1-phosphate, causing cellular damage and triggering the clinical manifestations of Classic Galactosemia. The enzyme GALT is encoded by the GALT gene, and mutations in this gene can result in the production of a non-functional enzyme.

**Why Each Wrong Option is Incorrect**
**Option A:** Hexosaminidase is an enzyme involved in the degradation of gangliosides, specifically GM2 gangliosidosis (Tay-Sachs disease), and is not related to galactose metabolism.
**Option B:** Glucocerebroside is a type of glycosphingolipid, and its deficiency leads to Gaucher's disease, a lysosomal storage disorder characterized by the accumulation of glucocerebroside in macrophages.
**Option C:** Sphingomyelinase is an enzyme involved in the degradation of sphingomyelin, and its deficiency leads to Niemann-Pick disease, a lysosomal storage disorder characterized by the accumulation of sphingomyelin in cells.

**Clinical Pearl / High-Yield Fact**
Classic Galactosemia can be diagnosed through a Guthrie test, which measures the level of galactose-1-phosphate in the blood. Early diagnosis and treatment can prevent the development of systemic complications, such as liver disease, cataracts, and intellectual disability.

**✓ Correct Answer: D. Galactose-1-Phosphate-Uridyl-Transferase**