**Core Concept**
Congenital toxoplasmosis occurs when a pregnant woman is infected with the Toxoplasma gondii parasite, which can cause severe and potentially life-threatening complications in the fetus. The manifestations of congenital toxoplasmosis are largely dependent on the timing of the maternal infection during pregnancy.

**Why the Correct Answer is Right**
The most common manifestation of congenital toxoplasmosis is chorioretinitis, which is an inflammatory condition of the choroid and retina in the eye. This is due to the parasite's ability to infect and cause damage to the developing retina. Chorioretinitis can lead to visual impairment and is a major concern in congenital toxoplasmosis. The parasite's invasion of the retina triggers an inflammatory response, which can cause scarring and lead to vision loss.

**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:** Mental retardation can occur in severe cases of congenital toxoplasmosis, but it is not the most common manifestation.
**Option C:** Ocular toxoplasmosis is a reactivation of the latent infection in the retina, which can occur years after the initial infection, and is not the most common manifestation in congenital toxoplasmosis.

**Clinical Pearl / High-Yield Fact**
A key aspect of managing congenital toxoplasmosis is early diagnosis and treatment, as this can significantly reduce the risk of long-term complications, including chorioretinitis and vision loss.

**Correct Answer: C. Chorioretinitis.**