**Core Concept**
Williams syndrome is a rare genetic disorder caused by a deletion on chromosome 7, leading to a characteristic set of clinical features. This condition is characterized by a distinct phenotype, including a friendly, outgoing personality, distinctive facial features, and a range of medical complications.

**Why the Correct Answer is Right**
The key features of Williams syndrome include a broad forehead, short nose, full cheeks, and a wide mouth. Patients often exhibit a friendly and outgoing personality, with a strong desire for social interaction. Additionally, individuals with Williams syndrome may have intellectual disability, heart problems, and distinctive speech patterns. The genetic deletion that causes Williams syndrome affects the elastin gene, which is crucial for the development of the cardiovascular system.

**Why Each Wrong Option is Incorrect**
**Option A:** Growth restriction in infancy is a feature of Williams syndrome, not a distinguishing characteristic. While growth restriction may occur, it is not a defining feature of the condition.
**Option B:** Developmental delay is a common feature of Williams syndrome, and individuals often exhibit intellectual disability. However, this option is not the correct answer, as developmental delay is indeed a characteristic of the condition.
**Option C:** Elastin gene deletion is a hallmark of Williams syndrome, leading to cardiovascular complications. This option is correct, but as the correct answer, it will be discussed in the Clinical Pearl section.

**Clinical Pearl / High-Yield Fact**
To remember the key features of Williams syndrome, recall the acronym "EFMTS": Elfin face, Friendly personality, Medical problems, Typical speech patterns, and Supravalvular aortic stenosis (a common cardiovascular complication).

**Correct Answer: C. Elastin gene deletion is a hallmark of Williams syndrome, leading to cardiovascular complications.**