**Core Concept**
Congenital rubella syndrome (CRS) is a collection of congenital anomalies that occur in infants whose mothers are infected with rubella virus during pregnancy. The most common defects associated with CRS are a result of the virus's impact on fetal development, particularly during the first trimester.

**Why the Correct Answer is Right**
The most common defect of congenital rubella syndrome is deafness. This is due to the virus's ability to infect and damage the developing inner ear, leading to sensorineural hearing loss. The virus can also affect the development of the auditory nerve, further contributing to hearing impairment. In addition, CRS is associated with other congenital anomalies, including cataracts, heart defects, and intellectual disability.

**Why Each Wrong Option is Incorrect**
**Option A:** Ocular involvement, such as cataracts, is a common feature of CRS, but it is not the most common defect.
**Option B:** While intellectual disability can occur in CRS, it is not the most common defect associated with the syndrome.
**Option C:** Congenital heart defects, such as patent ductus arteriosus, can occur in CRS, but they are not the most common defect.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the risk of congenital anomalies increases with the gestational age at which the mother contracts rubella. Therefore, it is essential for pregnant women to be vaccinated against rubella before pregnancy to prevent CRS.

**Correct Answer:** D. Deafness.