Which of the following is a symptom of major depressive disorder?

Loss of motivation is indeed a significant symptom of major depressive disorder. Individuals with this condition often experience a profound lack of interest or pleasure in activities that they once found enjoyable. This can manifest as difficulty in initiating tasks, a sense of apathy towards personal goals or responsibilities, and a general feeling of being emotionally drained.

In contrast, increased energy, heightened social interactions, and extreme excitement are not associated with major depressive disorder. These traits might be indicative of other conditions, such as mania or hypomania in bipolar disorder, rather than reflecting the characteristics of a major depressive episode. The presence of loss of motivation can heavily impact a person's daily functioning and overall quality of life, making it an essential symptom to recognize and address in clinical settings.