: Feelings of guilt or worthlessness, poor concentration, and recurrent thoughts of death or suicide.

: A more chronic form where a depressed mood lasts for at least two years. Theories of Pathogenesis

Major depressive disorder (MDD) is a chronic psychiatric condition characterized by persistent sadness, loss of interest (anhedonia), and physical symptoms that significantly impair daily life. By 2030, the World Health Organization projects it will be the leading cause of global disease burden.

: Significant changes in appetite or weight, sleep disturbances (insomnia or hypersomnia), and psychomotor agitation or retardation.

Because MDD is multifactorial, no single mechanism explains every case. Leading scientific hypotheses include: Major Depressive Disorder - StatPearls - NCBI Bookshelf

Diagnosis typically requires a persistently low mood or decreased pleasure in most activities for at least two weeks.