## **Core Concept**
The question presents a clinical scenario suggestive of a metabolic disorder. The key features include projectile vomiting, abdominal distension, diarrhea, bilateral cataracts in a 3-day-old male baby, and a urine test positive for reducing substances. These symptoms point towards an inborn error of metabolism.

## **Why the Correct Answer is Right**
The symptoms described, particularly the presence of bilateral cataracts and the urine being positive for Benedict's test (indicating the presence of reducing sugars), are highly suggestive of **Galactosemia**. Galactosemia is a genetic disorder that affects how the body processes a simple sugar called galactose. A deficiency in the enzyme galactose-1-phosphate uridyltransferase (GALT) leads to the accumulation of toxic substances when galactose is ingested. This condition presents with symptoms such as vomiting, diarrhea, abdominal distension, and failure to thrive in infancy. The presence of cataracts is a classic finding due to the accumulation of galactitol in the lens of the eye.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Without the specific details of the options provided, we can infer based on common metabolic disorders that conditions like **Phenylketonuria (PKU)** do not typically present with cataracts or a positive Benedict's test for reducing substances in the urine as primary diagnostic clues.
- **Option B:** Similarly, other metabolic disorders such as **Maple Syrup Urine Disease (MSUD)** present with neurological symptoms and a distinct odor in the urine but do not typically cause cataracts or a positive test for reducing substances.
- **Option C:** If this option were to represent another condition, without specifics, it's hard to directly refute, but conditions not directly linked to the ingestion of specific sugars or not causing cataracts and reducing substances in urine would be less likely.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **any infant with symptoms of a metabolic disorder, especially those with vomiting, failure to thrive, and unusual physical findings like cataracts, warrants an immediate investigation for inborn errors of metabolism**. Early diagnosis and dietary intervention can significantly improve outcomes for many of these conditions. The presence of reducing substances in the urine can help narrow down the differential diagnosis.

## **Correct Answer: D. Galactosemia**