**Core Concept**
The question is testing the knowledge of congenital infections and their effects on fetal development. Congenital infections occur when a virus is transmitted from the mother to the fetus during pregnancy, leading to various birth defects and complications.

**Why the Correct Answer is Right**
Rubella virus is responsible for causing congenital rubella syndrome (CRS) when a pregnant woman is infected. CRS is characterized by a triad of symptoms: cataracts, cardiac defects, and deafness. The virus can cause severe abnormalities in fetal development, particularly during the first trimester. The mechanisms behind CRS involve the virus's ability to infect and damage the developing fetus's organs, including the eyes, heart, and auditory system.

**Why Each Wrong Option is Incorrect**
* **Option A:** CMV (Cytomegalovirus) is another common cause of congenital infections, but it is not typically associated with the specific triad of cataracts, cardiac defects, and deafness. CMV can cause a range of symptoms, including microcephaly, developmental delays, and hearing loss.
* **Option B:** Toxoplasmosis is a parasitic infection that can cause congenital toxoplasmosis, leading to symptoms such as hydrocephalus, intracranial calcifications, and chorioretinitis. However, it is not typically associated with the specific triad mentioned in the question.
* **Option D:** Herpes simplex virus can cause congenital herpes, leading to symptoms such as skin lesions, encephalitis, and disseminated infection. However, it is not typically associated with the specific triad mentioned in the question.

**Clinical Pearl / High-Yield Fact**
Rubella virus is highly contagious and can be prevented through vaccination. Pregnant women who have not received the MMR vaccine should avoid exposure to individuals with rubella. If a pregnant woman is exposed to rubella, she should receive immediate medical attention to prevent transmission to the fetus.

**Correct Answer: B. Rubella virus**