Which mood disorder is characterized by episodes of intense sadness alternating with periods of mania?

Bipolar Disorder is characterized by episodes of intense sadness, known as depressive episodes, that alternate with periods of mania, which is marked by elevated mood, increased energy, and sometimes impulsive or risky behavior. This cyclical nature of mood changes distinguishes bipolar disorder from other mood disorders.

In contrast, Major Depressive Disorder involves prolonged periods of depressed mood without the occurrence of manic episodes. Dysthymic Disorder, now known as Persistent Depressive Disorder, is characterized by a chronic state of low mood lasting for at least two years but does not include the manic episodes seen in bipolar disorder. Seasonal Affective Disorder, on the other hand, is a type of depression that is linked to seasonal changes, particularly during winter months, and does not encompass manic episodes.

Thus, the defining feature of bipolar disorder is the presence of both depressive and manic episodes, making it the correct answer to the question.