Gumapang Ka sa Lusak (English: You Crawl in the Mud), also known internationally as Dirty Affair, is a 1990 Filipino political thriller crime drama film directed by Lino Brocka from a story and screenplay written by Ricardo Lee. Starring Dina Bonnevie, Eddie Garcia, Charo Santos, Christopher de Leon, and Allan Paule, the film tackles the abuse of power and evil acts committed by Edmundo Guatlo and his cruel wife Rowena in which Rachel, a movie actress who is his mistress, and her friends try to expose his scandals and violent acts done by him and his wife and even his henchmen to the public. This was one of the two films of that year where Brocka collaborated with cinematographer Pedro Manding Jr. before the latter's murder; the other is Biktima.

Produced and distributed by Viva Films, the film was theatrically released on May 17, 1990, and was later screened through international film festivals including in France and Japan in 1994. In 2010, the film was remade into a television drama series of the same name by GMA Network as the nineteenth installment of Sine Novela and starred Dennis Trillo as Levi, Jennylyn Mercado as Rachel, Al Tantay as Edmundo Guatlo, and Sandy Andolong as Rowena Guatlo.