**Core Concept**
Leukocoria, also known as "cat's eye reflex," is a white pupillary reflex that can be observed in newborns. This condition is often a sign of an underlying ocular or systemic disease.

**Why the Correct Answer is Right**
Congenital cataract is the most common cause of leukocoria in newborns. This is because cataracts can cause light to scatter in the lens, resulting in a white pupillary reflex. Congenital cataracts are often bilateral and can be caused by various factors, including genetic mutations, maternal infections, or exposure to certain medications during pregnancy. The presence of a congenital cataract can lead to amblyopia (lazy eye) if left untreated.

**Why Each Wrong Option is Incorrect**
**Option B:** Toxocara infection is a parasitic infection that can cause ocular larva migrans, but it is not the most common cause of leukocoria in newborns.

**Option C:** Retinoblastoma is a malignant tumor of the retina that can cause leukocoria, but it is relatively rare in newborns and typically presents later in childhood.

**Option D:** CMV infection can cause congenital infections, including retinitis, which can lead to leukocoria, but it is not the most common cause in newborns.

**Clinical Pearl / High-Yield Fact**
A congenital cataract can lead to amblyopia if left untreated, emphasizing the importance of prompt ophthalmological evaluation in newborns with leukocoria.

**✓ Correct Answer: A. Congenital cataract**