Which disorder would involve a person feeling extreme sadness and a lack of interest in activities they once enjoyed?

The described characteristics of extreme sadness and a lack of interest in previously enjoyable activities align very well with Major Depression. This disorder is characterized by persistent feelings of sadness or a low mood and a significant decrease in the interest or pleasure in most activities, known as anhedonia. Individuals with Major Depression often experience a variety of symptoms that can include fatigue, changes in appetite, sleep disturbances, and difficulty concentrating, all of which can severely impact their daily functioning.

The distinction of Major Depression lies in its focus on mood and motivation, making it very different from schizophrenia, which involves symptoms such as delusions and hallucinations, and bipolar disorder, which includes fluctuating periods of both depressive and manic or hypomanic episodes. Somatic Symptom Disorder is characterized primarily by a focus on physical symptoms that cause significant distress or impairment, without the core mood-related symptoms seen in Major Depression. Thus, the symptoms specified in the question are unmistakably indicative of Major Depression.