Bipolar mood disorder is distinguished from major depression by:

Bipolar mood disorder is characterized by the presence of at least one episode of mania or a manic episode. This is a key distinguishing factor between bipolar disorder and major depression. Major depression involves episodes of depressive symptoms without the occurrence of manic or hypomanic episodes. The manic phase in bipolar disorder includes symptoms such as elevated mood, increased energy, reduced need for sleep, and heightened impulsivity, which are not present in major depressive episodes.

The other choices center around different types of mood disorders or specific histories that do not define the essential difference between bipolar disorder and major depression. For instance, earlier instances of cyclothymia (a milder form of mood instability) or dysthymia (persistent depressive disorder) are not defining factors of bipolar disorder. Additionally, chronic forms of depression may present a more complex picture of mood disorders but do not inherently separate bipolar disorder from major depression. Thus, the presence of at least one manic episode is the hallmark criterion that clearly distinguishes bipolar mood disorder from major depressive disorder.