**Core Concept**
Galactosemia is a genetic disorder that affects the body's ability to metabolize galactose, a sugar found in milk. This leads to the accumulation of galactose and its byproduct, galactitol, in various tissues, causing damage and symptoms.

**Why the Correct Answer is Right**
Infants with galactosemia who are fed breast milk or formula containing lactose (a disaccharide composed of glucose and galactose) develop symptoms such as vomiting, diarrhea, and failure to thrive. The accumulation of galactitol in the lens of the eye causes cataract formation. This is due to the inhibition of galactose-1-phosphate uridyltransferase, an enzyme essential for the conversion of galactose to glucose.

**Why Each Wrong Option is Incorrect**
* **Option A:** Congenital rubella syndrome can cause cataract and other ocular abnormalities, but it is not associated with intolerance to breast milk or diarrhea.
* **Option B:** Congenital hypothyroidism can cause cataract, but it is not typically associated with gastrointestinal symptoms.
* **Option C:** Congenital adrenal hyperplasia can cause vomiting and diarrhea, but it is not typically associated with cataract formation.

**Clinical Pearl / High-Yield Fact**
Galactosemia is a classic example of a genetic disorder that can be prevented by avoiding lactose-containing substances in the diet. Newborn screening programs often include a test for galactosemia to identify affected infants early.

**Correct Answer:** C. Galactosemia.