**Core Concept**
Galactosemia is a congenital metabolic disorder characterized by the inability to metabolize galactose, a sugar found in milk, leading to its accumulation in the body. This condition is typically identified through the presence of reducing sugars in the urine, which can be detected using the Benedict's test.

**Why the Correct Answer is Right**
In galactosemia, the enzyme galactose-1-phosphate uridyltransferase (GALT) is deficient or defective, preventing the conversion of galactose to glucose. As a result, galactose is shunted into the pentose phosphate pathway, where it is converted to galactose-1-phosphate. This intermediate is then excreted in the urine, where it can be detected as a reducing sugar using the Benedict's test. The presence of galactose in the urine is a hallmark of galactosemia.

**Why Each Wrong Option is Incorrect**
**Option A:** Glucose is not typically present in the urine of a normal newborn, but it can be detected in the urine of individuals with diabetes mellitus. However, the clinical presentation of a 3-day-old baby with feeding difficulties and vomiting is more suggestive of galactosemia than diabetes mellitus.

**Option B:** Fructose is a reducing sugar, but it is not typically associated with the clinical presentation described. Fructosemia is a rare condition that involves the inability to metabolize fructose, but it is not as common as galactosemia.

**Option C:** Lactose is a disaccharide composed of glucose and galactose. While lactose is present in milk, it is not typically detected in the urine of a newborn with galactosemia.

**Clinical Pearl / High-Yield Fact**
The presence of a reducing sugar in the urine of a newborn, especially one who is feeding poorly and vomiting, should raise suspicion for galactosemia. A definitive diagnosis can be made through a galactose-1-phosphate uridyltransferase (GALT) assay or a galactose challenge test.

**Correct Answer:** C. Galactose