**Core Concept:** Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder characterized by impairments in social interaction, communication, and repetitive and restricted behavior. Normal milestones and lack of interaction with others are key features.

**Why the Correct Answer is Right:** The correct answer is A (Autism Spectrum Disorder) because it encompasses the described clinical features: normal developmental milestones and lack of social interaction with others. ASD affects the central nervous system, leading to impairments in social communication and behavior.

**Why Each Wrong Option is Incorrect:**
A. Developmental Disorder of Cortical Development (DDCD) is not the correct answer because it is a rare cause of developmental delay and not specifically related to social interaction problems.
B. Rett Syndrome is a specific form of ASD affecting females, characterized by regression, seizures, and stereotypic hand movements. While this might seem like a correct answer due to the regression aspect, the lack of social interaction and normal milestones make ASD the most fitting diagnosis.
C. Down syndrome is a chromosomal disorder with intellectual disability and distinctive facial features, not related to social interaction problems and normal milestones.
D. Williams Syndrome is a genetic disorder with characteristic facial features, intellectual disability, and social interaction issues. Although it is a correct answer in some aspects, the lack of regression and normal milestones distinguish ASD as the more fitting diagnosis.

**Clinical Pearl:** Autism Spectrum Disorder is a complex condition that can be challenging to diagnose, as it encompasses a wide range of symptoms and behaviors. Early identification and intervention are crucial for improving outcomes. Families and healthcare providers should be aware of the signs and symptoms of ASD in children, including normal milestones and social interaction problems.