The primary developmental disability is mental retardation. Mental retardation is defined as significant cognitive (IQ less than 70) and functional delays with onset before adulthood. Over 40 percent of individuals with mental retardation have associated medical conditions. Typically the lower the patient's IQ, the more likely a specific etiology of the mental retardation can be identified. Etiologic factors implicated in mental retardation may be prenatal, perinatal, postnatal, or traumatic. Genetic factors account for 7 to 15 percent of all mental retardation; there are 500 genetic syndromes known to be associated with mental retardation. Most of these genetic syndromes are multisystem disorders with a large number of associated medical problems. Cardiac, orthopedic, connective tissue, and neurologic disorders are particularly common.3 Down syndrome (trisomy 21) is the most common genetic cause of mental retardation, accounting for 4 to 12 percent of all mental retardation, with an incidence of 1/1000 live births.4 Fragile X is the most common single-gene disorder and accounts for 1 to 6 percent of all mental retardation. The incidence of fragile X syndrome in males is 1/1000 with almost all affected individuals having mental retardation. In females the incidence of fragile X syndrome is 1/2000, but less than half of females with fragile X syndrome have mental retardation. Other common genetic etiologies include Prader-Willi syndrome and Williams syndrome, each with a prevalence in live births of 0.1/1000, Lesch-Nyhan syndrome, with an estimated prevalence of 0.02/1000, and phenylketonuria (PKU), with a prevalence of 0.08/1000.5 Exposures to toxins may cause as much as 15 percent of mental retardation with fetal alcohol syndrome (FAS) responsible for about 10 percent of all mental retardation.6 Depending on the etiology, some medical problems are associated with specific mental retardation syndromes ( T,aMe.3.0.2,-,1).
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.