**Core Concept**
Galactosemia is a genetic disorder characterized by the inability to metabolize galactose due to a deficiency in the enzyme galactose-1-phosphate uridyltransferase (GALT). This leads to the accumulation of galactose-1-phosphate in the body, causing various complications, including cataract formation.

**Why the Correct Answer is Right**
The accumulation of galactose-1-phosphate in galactosemia leads to the inhibition of the enzyme aldose reductase, which normally converts glucose to sorbitol. However, in the presence of galactose-1-phosphate, this enzyme is inhibited, causing an increase in the levels of galactitol in the lens of the eye. The accumulation of galactitol in the lens leads to the formation of cataracts due to the increase in osmotic pressure and subsequent swelling of the lens fibers.

**Why Each Wrong Option is Incorrect**
**Option A:** Glucose-6-phosphate dehydrogenase (G6PD) deficiency is associated with hemolytic anemia and not cataract formation.
**Option B:** Fructose-1,6-bisphosphate is an intermediate in glycolysis and not directly related to cataract formation in galactosemia.
**Option C:** Urea cycle disorders are associated with the accumulation of ammonia and not galactose-1-phosphate.

**Clinical Pearl / High-Yield Fact**
Galactosemia patients can develop cataracts within the first few weeks of life, emphasizing the need for early diagnosis and treatment to prevent long-term complications.

**Correct Answer: C. Galactitol.