What defines bipolar and related disorders?

Bipolar and related disorders are characterized by significant shifts between emotional extremes. This includes the experience of episodes of mania or hypomania, which are marked by elevated mood, increased energy, or irritability, as well as depressive episodes where individuals may feel profound sadness or loss of interest. The hallmark of bipolar and related disorders is this fluctuation between these high and low mood states, making the presence of such shifts the defining feature.

In contrast, the other options do not accurately capture the essence of bipolar disorders. The presence of only depressive episodes does not align with the diagnostic criteria for bipolar disorders, which require the presence of both manic/hypomanic and depressive episodes over time. Moreover, stating that only hypomanic episodes are present would fail to recognize the full spectrum of mood changes essential to the diagnosis. Finally, a consistency in mood without extremes would describe a different profile entirely, such as a mood disorder that does not involve fluctuations like those seen in bipolar conditions. Hence, the correct choice reflects the critical aspect of emotional variability that defines bipolar and related disorders.