Someone who alternates among several distinct personalities, each having different abilities, different memories, and perhaps even different names, is suffering from

A) obsessive-compulsive disorder.
B) schizophrenia.
C) manic-depressive disorder.
D) dissociative identity disorder.

This is a classic description of dissociative identity disorder, formerly known as multiple personality disorder. Obsessive-compulsive disorder involves persistent, intrusive thoughts and repetitive behaviors, while schizophrenia involves a broad range of symptoms including delusions, hallucinations, disorganized speech, and disorganized behavior. Manic-depressive disorder, also known as bipolar disorder, involves mood swings between episodes of depression and episodes of mania. While dissociative identity disorder is controversial and its diagnosis remains somewhat controversial, it is recognized as a legitimate diagnosis by the American Psychiatric Association.