For the past four weeks, Nan has been feeling lethargic and worthless. Her friends are worried because she no longer shows interest in her normal social activities. A clinician using the DSM-5 would MOST likely diagnose Nan as suffering from .

The answer is major depressive disorder.

Explanation:

Major depressive disorder is a mental condition characterised by low mood, profound sadness and constant feelings of pain, often inexplicably. This disorder causes people to have mental distortions, meaning they have false or unrealistic beliefs about the world or the people around them.

People who suffer from this disorder are greatly affected in different areas of their life, especially family, school, the workplace and social relationships. It may be treated with therapy and antidepressant medication.