**Core Concept**
Autistic spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impaired social interaction, communication, and repetitive behaviors. The prognosis of ASD varies widely among individuals, depending on the severity and type of symptoms, as well as the presence of co-occurring conditions.

**Why the Correct Answer is Right**
High-functioning autism (HFA) is a subtype of ASD that is characterized by a relatively normal cognitive and adaptive functioning. Individuals with HFA may have difficulties with social interactions and repetitive behaviors, but they typically do not have significant impairments in language or cognitive skills. This subtype of ASD is often associated with a better prognosis compared to other forms of ASD, as individuals with HFA are more likely to develop compensatory strategies and adapt to their environment. Research suggests that individuals with HFA may have a more favorable outcome in terms of academic and vocational achievement, as well as social relationships.

**Why Each Wrong Option is Incorrect**
**Option A:** Asperger's syndrome is a subtype of ASD that is characterized by difficulties with social interactions and repetitive behaviors, but without significant impairments in language or cognitive skills. However, it is not generally associated with a better prognosis compared to HFA.
**Option B:** Pervasive developmental disorder not otherwise specified (PDD-NOS) is a subtype of ASD that is characterized by a mixture of symptoms from different subtypes. It is not typically associated with a better prognosis compared to HFA.
**Option C:** Rett syndrome is a rare genetic disorder that affects brain development and is characterized by severe impairments in language, cognitive, and motor skills. It is not a subtype of ASD, but rather a separate neurodevelopmental disorder.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the prognosis of ASD can vary widely among individuals, and that each person with ASD should be evaluated on a case-by-case basis. A comprehensive diagnostic evaluation and ongoing support are critical for improving outcomes for individuals with ASD.

**Correct Answer: C. Rett syndrome is a rare genetic disorder that affects brain development and is characterized by severe impairments in language, cognitive, and motor skills.**