posted by Mariah .
Will just taking out the "it" make this sentence correct?
The disease is typified by a delay in motor development, by self-destructive behavior, and it leads to death.
No, in order to make the series' elements parallel, you need a third phrase that starts with "by" and ends with a prepositional phrase ... so you have this:
... by a delay ..., by self-destructive behavior, and by ________________.
How will you rephrase the last part of this sentence?
and by death.
Yes, that works. Good!