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What is Intense nihilism, somatization and agitation in old age are the hallmark symptoms of - and explain in detail?
Involutional melanocholia has following featurs

i)    Involutional (old) age group
ii) Melanocholia (multiple somatic symptoms → somatization)
iii) Psychomotor agitation / Retardation
iv) Psychotic features (delusion of nilism, guilt, perseution, Hypochondriacal; Hallucinations).

Atypical depression is characterized by increased appetite / increased weight, increased sleep (hypersomnia), Lethargy, Rejection sensitivity.
For somatized depression see a previous explanation.
Depressive stupor occurs in the severe depression where the person is akinetic (immobile) and mute.