A 4-year-old girl who is an only child stas school. He has never shown much of an interest in interacting with others. He has never used many words which the parents attribute to his isolation as an only child. They tell the teacher he has always done best with a strict schedule and doesn’t tolerate changes well. Other children attempt to play with him but his response is to either ignore them or get angry at their advances. The patient described can best be distinguished from Rett disorder because of which of the following?

**Core Concept**
The question is testing the distinction between Autistic Disorder (also known as Autism Spectrum Disorder) and Rett Disorder, two neurodevelopmental disorders characterized by social and communication impairments. The key differentiating factor lies in the developmental trajectory and associated clinical features.

**Why the Correct Answer is Right**
Autistic Disorder is characterized by persistent deficits in social communication and social interaction across multiple contexts, including deficits in nonverbal communicative behaviors used for social interaction, such as eye contact and facial expressions. Additionally, individuals with Autistic Disorder may exhibit restricted, repetitive patterns of behavior, interests, or activities. In contrast, Rett Disorder is a disorder primarily affecting girls, characterized by a distinctive pattern of regression in language and social skills, followed by a period of stagnation and then a slow decline. The core feature of Rett Disorder is the loss of purposeful hand use, which is not seen in Autistic Disorder.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while both disorders involve social communication impairments, the loss of purposeful hand use is a hallmark of Rett Disorder, not Autistic Disorder.
**Option B:** This option is incorrect because both disorders involve restricted, repetitive behaviors, but the pattern and severity differ between the two disorders.
**Option C:** This option is incorrect because both disorders involve significant developmental delays, but the nature and timing of these delays differ between the two disorders.

**Clinical Pearl / High-Yield Fact**
In clinical practice, it's essential to differentiate between Autistic Disorder and Rett Disorder, as the two disorders have distinct prognoses and treatment approaches. A thorough developmental history and physical examination can help differentiate between the two disorders.

**Correct Answer: D.**

✓ Correct Answer: C. Absence of a period of normal development