**Core Concept**
Brushfield spots are small, white or grayish spots on the periphery of the iris. They are a benign condition but have been associated with various genetic conditions, including Down syndrome, congenital rubella, and other chromosomal abnormalities.

**Why the Correct Answer is Right**
Brushfield spots are caused by the presence of small white or grayish deposits on the iris, which are thought to be related to the abnormal development of the iris stroma. These deposits can be seen in individuals with Down syndrome, where they are present in approximately 70% of cases. The exact mechanism is not fully understood, but it is believed to be related to the genetic abnormalities associated with Down syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Brushfield spots are not typically associated with conditions such as myotonic dystrophy, which is a different genetic disorder.
**Option B:** This option is incorrect because while congenital rubella can cause various eye abnormalities, Brushfield spots are not a characteristic feature of this condition.
**Option C:** This option is incorrect because while chromosomal abnormalities can cause various eye abnormalities, Brushfield spots are not typically associated with conditions such as Turner syndrome.

**Clinical Pearl / High-Yield Fact**
Brushfield spots are an important clinical finding in individuals with Down syndrome, and their presence can help support a diagnosis of this condition. They are also an important consideration in individuals with a family history of Down syndrome, as they may indicate an increased risk of having a child with the condition.

**Correct Answer: D. Down syndrome.**