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Upton Sinclair is primarily known for the genre of muckraking, a style of investigative journalism that exposes corruption, exploitation, and social injustice. Through his novels and articles, Sinclair aimed to shed light on the harsh realities faced by the working class and the abuses of power by corporations and political figures. His works, such as "The Jungle" and "The Brass Check," contributed significantly to the Progressive Era's reform movements.