Which of the following is a symptom of Major Depressive Disorder?

The symptom of persistent sadness and loss of interest is characteristic of Major Depressive Disorder (MDD). Individuals experiencing MDD often report a notable decline in their mood, feeling sad or empty most of the day, nearly every day. Alongside this pervasive sadness, there is usually a significant loss of interest or pleasure in activities that were once enjoyable, known clinically as anhedonia. This symptom reflects the profound impact depression has on a person's emotional and motivational state, often leading to difficulties in daily functioning.

The other options reflect experiences that are generally contrary to what individuals with MDD undergo. Increased energy and happiness, heightened focus and concentration, and extreme optimism are not associated with depressive disorders; rather, they suggest a more elevated or stable emotional state which is not typical of those suffering from major depression.