## **Core Concept**
The clinical presentation of a 3-day-old child vomiting everything fed, having a distended abdomen, diarrhea, and urine positive for Benedict's test for reducing substances, suggests a condition related to carbohydrate metabolism. Benedict's test is used to detect the presence of reducing sugars, which include substances like glucose, fructose, lactose, and galactose.

## **Why the Correct Answer is Right**
The symptoms described are indicative of galactosemia, a condition where the body is unable to properly break down galactose, a sugar found in milk and other dairy products. This condition arises due to a deficiency of the enzyme **galactose-1-phosphate uridyltransferase**, which is necessary for the metabolism of galactose. As a result, galactose accumulates in the body and is excreted in the urine. Galactose is a reducing sugar and will test positive with Benedict's test.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because the substance in question is not specified, but based on the context, it does not directly relate to the condition described.
- **Option B:** This option suggests glucose, but while glucose is a reducing sugar, the clinical context provided (vomiting, distended abdomen, diarrhea in a newborn) and the specific issue with feeding (which could imply lactose or galactose issue) points more towards galactosemia than a simple glucose issue.
- **Option C:** This could potentially be a correct mechanism but specifying the wrong substance.
- **Option D:** This option is incorrect because, although lactose is a disaccharide made of glucose and galactose, the direct substance detected in urine due to a metabolic issue would be galactose, not lactose itself.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that galactosemia presents in newborns with symptoms such as vomiting, failure to thrive, hepatomegaly, and jaundice after feeding with lactose-containing milk. Early diagnosis and dietary restriction of galactose-containing foods are crucial to prevent long-term complications, including intellectual disability and liver disease.

## **Correct Answer:** . Galactose