A patient goes to her primary care doctor complaining of mood swings affecting her in such a way that she often feels either very "high" or very "low." She denies any thoughts of worthlessness or of hurting herself, doesn't have trouble concentrating on tasks, and has no psychotic episodes when she is "low." When feeling "high," she has increased energy and happiness. Her symptoms are indicative of which of the following mood disorders?

Lithium is a medication that is effective in the treatment of __________.