Which syndrome is female-only with onset between 6 and 18 months, followed by motor deterioration and loss of language, often with epilepsy and hand stereotypes?

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This item is testing recognition of Rett syndrome. The pattern—normal early development followed by a regression between about 6 and 18 months, with loss of purposeful hand skills and language, and the emergence of repetitive hand-wringing or hand-stereotypies, along with motor deterioration and often epilepsy—is characteristic of Rett syndrome. It is nearly exclusively seen in females because it is caused by an X-linked dominant mutation (MECP2); males with the mutation are much less likely to have the classic Rett course.

Understanding why this fits helps distinguish it from the other conditions. Angelman syndrome involves developmental delay, ataxia, and often a happy demeanor with frequent laughter and seizures, but it does not feature the Rett-style regression with hand-wringing beginning in that 6–18 month window. Fragile X syndrome is inherited on the X chromosome and can affect both sexes, with features like hand flapping and autism-like behaviors, but it does not present with the female-only Rett regression pattern. Prader-Willi syndrome starts with hypotonia in infancy and later leads to hyperphagia and obesity, again without the Rett-like loss of language and hand skills with stereotypic hand movements.