## **Core Concept**
The question describes a newborn presenting with symptoms such as lethargy, refusal to feed, vomiting, and a pot-bellied appearance a few days after birth. These symptoms, combined with a positive Benedict's test indicating the presence of a reducing substance in the urine, point towards a metabolic disorder. The most likely condition here is **Galactosemia**, which is a disorder of carbohydrate metabolism.

## **Why the Correct Answer is Right**
Galactosemia is caused by a deficiency of the enzyme **galactose-1-phosphate uridyltransferase** (GALT), which is necessary for the metabolism of galactose, a sugar found in milk and other dairy products. When this enzyme is deficient, galactose accumulates in the body and is excreted in the urine. The symptoms described can be attributed to the toxic effects of galactose and its metabolites. A positive Benedict's test in this context suggests the presence of **galactose** as the reducing substance in the urine.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, although glucose is a reducing sugar, the clinical presentation does not align with simple glucose metabolism disorders like diabetes mellitus, which would not typically present with these symptoms in a newborn.
- **Option B:** This option is incorrect because, while lactose is a sugar found in breast milk, it is not directly excreted in urine; rather, its components (glucose and galactose) are either absorbed or metabolized.
- **Option D:** This option is incorrect because fructose is another simple sugar, but the symptoms and the metabolic pathway involved do not align with a primary issue of fructose metabolism in this context.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that galactosemia presents with symptoms shortly after birth, once milk feeding starts, and includes failure to thrive, hepatomegaly (which could contribute to the pot-bellied appearance), and often **cataracts** due to the accumulation of galactitol in the lens of the eye. Early diagnosis through newborn screening is crucial for management and prevention of long-term complications.

## **Correct Answer:** . Galactose