**Core Concept**
Congenital toxoplasmosis is a condition where the Toxoplasma gondii parasite is transmitted from the mother to the fetus during pregnancy. This can lead to various manifestations in the newborn, depending on the timing and severity of the infection.

**Why the Correct Answer is Right**
The most common manifestation of congenital toxoplasmosis is chorioretinitis, which is an inflammation of the choroid and retina in the eye. This occurs due to the parasite's invasion of the retina, leading to damage and scarring. Chorioretinitis can cause vision loss and is often the primary reason for the diagnosis of congenital toxoplasmosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Hydrocephalus is a potential complication of congenital toxoplasmosis, but it is not the most common manifestation.

**Option B:** Seizures can occur in infants with congenital toxoplasmosis, especially if the infection is severe or if there is associated brain involvement, but they are not the most common manifestation.

**Option D:** Cerebral calcifications are a common finding in congenital toxoplasmosis, but they are not the most common manifestation.

**Clinical Pearl / High-Yield Fact**
Chorioretinitis is often asymptomatic in the neonatal period but can cause vision loss if left untreated. Early diagnosis and treatment are essential to prevent long-term complications.

**Correct Answer: C. Chorioretinitis.**