**Question:** Defect most commonly occurring in congenital rubella is -

A. Eye abnormalities
B. Heart defects
C. CNS defects
D. Immunodeficiency

**Core Concept:** Congenital rubella syndrome (CRS) is a complex disorder resulting from maternal rubella infection during pregnancy. Rubella virus primarily targets the developing fetus, leading to a range of organ system abnormalities.

**Why the Correct Answer is Right:** The defect most commonly occurring in congenital rubella is cardiovascular abnormalities, specifically heart defects. These defects include patent ductus arteriosus, coarctation of aorta, ventricular septal defect, and atrial septal defect. The rubella virus can infect the cardiovascular system during pregnancy, leading to the observed heart defects.

**Why Each Wrong Option is Incorrect:**

**A. Eye abnormalities:** While eye abnormalities can occur in CRS, they are not the most common manifestation. The most common eye defects include cataracts, microphthalmia, and optic nerve hypoplasia.

**B. Heart defects:** As mentioned earlier, heart defects are the most common, followed by eye abnormalities. CNS defects and immunodeficiency are less common.

**C. CNS defects:** While CNS defects can occur in CRS, they are not the most common manifestation. The most common CNS defects include microcephaly, hydrocephalus, and seizures.

**D. Immunodeficiency:** Immunodeficiency is a less common manifestation of CRS, affecting approximately 10-20% of affected infants. Heart defects and eye abnormalities are more common.

**Clinical Pearl:** The knowledge of common manifestations of congenital rubella syndrome is crucial for healthcare providers to identify and manage affected infants appropriately. Timely diagnosis and treatment can prevent or mitigate the adverse effects of CRS on the affected infant's health.

**Correct Answer:** B. Heart defects