I read some newspaper:
"His lawyers argued that he was not guilty by reason of mental defect, and a defense psychiatrist testified that Muhammad was delusional."
Shouldn't there be some article before "mental defect", since "defect" is countable?
Good question. Very observant.
In this case, however, "not guilty by reason of mental defect" is a defense term taken from a written penal code. It is a specific term unto itself and no article is necessary.