Okay, let's tackle this question step by step. The patient is a 3-week-old neonate with vomiting after breastfeeding, abdominal distension, liver enlargement, and jaundice. The urine tests are negative for glucose but positive for reducing sugar. Hmm, what conditions could present like this?

First, the symptoms: vomiting after feeding, abdominal distension, hepatomegaly, and jaundice. These are classic signs of a metabolic disorder. The key here is the urine test showing reducing sugars but no glucose. That suggests galactosemia, because galactose, a reducing sugar, would be present in the urine when glucose isn't. Galactosemia is a genetic disorder where the body can't process galactose properly, leading to accumulation and toxicity.

Let me think about the options. If the answer is galactosemia, then the other options would be other metabolic or liver disorders. For example, neonatal hepatitis or biliary atresia might cause jaundice and hepatomegaly but wouldn't have the reducing sugar in urine. Congenital hypothyroidism could lead to jaundice and abdominal distension but not the urine findings. Another possibility might be a galactokinase deficiency, but that's a milder form and might not present with such severe symptoms.

The urine dipstick test is crucial here. Reducing sugars can be from galactose, lactose, or other reducing agents. Since glucose is negative, galactose is the likely culprit. Galactosemia is autosomal recessive, and the enzyme deficiency is usually galactose-1-phosphate uridyltransferase (GALT) in classic galactosemia. The accumulation of galactose and its metabolites causes the symptoms mentioned.

So the correct answer should be galactosemia. The other options don't fit the urine test results. The clinical pearl here is to remember that reducing sugars in urine without glucose is a hallmark of galactosemia, especially in a neonate with these symptoms. That's a key point for exams.

**Core Concept**
This question tests recognition of **classic galactosemia**, an autosomal recessive metabolic disorder caused by deficiency of **galactose-1-phosphate uridyltransferase (GALT)**. Key features include neonatal jaundice, hepatomegaly, vomiting, and **urinary reducing substances without glucose** due to galactose excretion.

**Why the Correct Answer is Right**
In galactosemia, undigested galactose from breast milk is excreted in urine. Urine dipstick tests detect **reducing sugars** (e.g., galactose) via the **Clinitest** method, but **glucose oxidase-based tests** (used in standard dipsticks) fail to detect galactose. The clinical triad of **vomiting, jaundice, and hepatomegaly** in a neonate with reducing substances in urine confirms the diagnosis. Untreated, it causes liver failure, cataracts, and intellectual disability.

**Why Each Wrong Option is Incorrect**
**Option A:** *Neonatal hepatitis* causes jaundice and hepatomegaly but lacks urinary reducing sugars.
**Option B:** *Galactokinase deficiency* (a milder galactose metabolism defect) causes cataracts but

✓ Correct Answer: B. Classic galactosemia