What would cause a fifteen point drop in an IQ test?

A fifteen point drop in IQ could be caused by a number of factors. Some of these include:

a severe head injury

a stroke or brain aneurysm

severe mental illness such as schizophrenia

dementia

brain tumor

lead poisoning

*other serious medical conditions such as diabetes or heart disease

substance abuse

*long term stress

*traumatic brain injury

*certain neurological conditions

*poor nutrition