Difference between contusion and post-moem lividity is that post-moem lividity-
Shows sequential color change
Easily washable
Shows diffuse irregular margins
Has raised enzyme levels
The synopsis of forensic medicine & toxicology;Dr k.s narayan reddy; 28th edition;pg.no. 88; Contusion or bruises are not washable but postmoem lividity is washable. Also refer page no.106
Mechanical injuries