What is characterized by disruptions in mood and is marked by low mood only?

Persistent depressive disorder, also known as dysthymia, is characterized by a chronic low mood that lasts for at least two years in adults (one year in children and adolescents). This condition primarily emphasizes prolonged periods of depression without the more severe and episodic disruptions seen in major depressive disorder. Individuals with persistent depressive disorder often experience a consistent sense of sadness or a lack of interest in activities, which can impact their daily functioning and overall quality of life.

Unlike major depressive disorder, where individuals may experience distinct episodes of depression that can vary in intensity, persistent depressive disorder maintains a steady low mood over an extended period. Cyclothymia involves mood fluctuations but includes periods of hypomania, and premenstrual dysphoric disorder is specifically linked to the menstrual cycle and includes both mood disturbances and physical symptoms. Therefore, the hallmark of persistent depressive disorder is the consistent presence of low mood over time, which fits the description provided in the question.