The more attributes you add in, the more difficult it'll be to find a disease that matches. Also, if you're going for maximum realism, you'll need to think of a lot more than just the symptoms. A genetic disease, by definition, is present at birth and if it affects either physical or mental growth, the signs will appear very early on in the child's life - so you'll have to have a back story to explain how/why the parents/doctors allowed the child to end up in whatever situation he/she now finds him/herself.
If you're dead-set on a genetic disease, this might create an inherent problem whereby one or both of the parents should be carriers of the gene, in which case you'll then need to double check the disease profile to see if they should have had identifiable problems.
You could take the easy way out and have had the foetus suffer ante-natal damage (mother exposed to radiation, drugs, some weird environmental concoction) ... but again, if you're going for maximum realism, you'll have to explain how/why no-one else had the same problem - and how the child survived to this age. Most of these multi-problem defects are pretty fatal to the individual.