Mental Retardation: Clinical Classification of Mental Retardation are as follows: 1. Mongolism 2. Cretinism 3. Microcephaly 4. Hydrocephaly 5. Phenylketonuria (PKU) 6. Turner Syndrome. Mental retardation can be classified not only on the basis of the level of intelligence, but also on the basis of their clinical symptoms. (more…)