**Core Concept**
The question is testing the understanding of the teratogenic effects of various viral infections during pregnancy, specifically in the first trimester. Rubella, Toxoplasmosis, and CMV are all known to cause congenital malformations, but the extent of their teratogenic effects varies.

**Why the Correct Answer is Right**
Rubella virus is particularly notorious for its teratogenic effects, especially if infection occurs during the first trimester. The virus can cause congenital rubella syndrome (CRS), which is characterized by a range of congenital malformations, including deafness, cataracts, heart defects, and intellectual disability. This is because the virus can infect the developing fetus and disrupt the normal development of various organs and tissues. The mechanism of rubella-induced teratogenesis is thought to involve the virus's ability to infect and replicate in the placenta, leading to the release of inflammatory cytokines that damage the developing fetus.

**Why Each Wrong Option is Incorrect**
* **Option B:** Toxoplasmosis can cause congenital toxoplasmosis, which is characterized by chorioretinitis, hydrocephalus, and intracranial calcifications. However, the incidence of congenital malformations due to Toxoplasmosis is lower than that of Rubella.
* **Option C:** CMV is a leading cause of congenital infection, but its teratogenic effects are generally less severe than those of Rubella. CMV infection during pregnancy is associated with a range of congenital malformations, including hearing loss, developmental delay, and vision impairment.
* **Option D:** While all three infections can cause congenital malformations, Rubella has the maximum incidence of congenital malformations if it occurs in the first trimester.

**Clinical Pearl / High-Yield Fact**
The key takeaway from this question is that Rubella infection during the first trimester has the highest risk of congenital malformations, making it a critical target for vaccination efforts, particularly in pregnant women.

✓ Correct Answer: A. Rubella