Which one of the following is not a neurologic cause of autistic symptoms?

The identification of Marfan's syndrome as not a neurologic cause of autistic symptoms is rooted in the understanding of the underlying mechanisms of the conditions mentioned. Marfan's syndrome is primarily a connective tissue disorder that affects the skeletal, cardiovascular, and ocular systems. While individuals with Marfan's syndrome may exhibit some neuropsychiatric symptoms, the core features of this disorder do not include autism spectrum disorder (ASD) or its characteristic behavioral symptoms.

In contrast, Rett's syndrome, Fragile X syndrome, and Angelman's syndrome are all associated with neurodevelopmental disorders and can manifest with autistic symptoms. Rett's syndrome primarily affects females and involves regression of developmental skills, loss of purposeful hand use, and distinctive gait abnormalities, often accompanied by autistic features. Fragile X syndrome is the most common inherited form of intellectual disability and is known to have a strong correlation with autism. Similarly, Angelman's syndrome, characterized by severe developmental delays, speech impairment, and gait ataxia, also frequently presents with behaviors commonly found in autism.

Thus, the association of the first three syndromes with neurologic causes of autistic symptoms is well-established, while Marfan's syndrome does not primarily involve neurodevelopmental impacts, making it the correct choice in this context.