Peritonitis

A 30-year-old man is brought to the emergency department for abdominal pain and tenderness that have been worsening over the past seven hours. He rates the pain as an 8 on a 10-point scale, and says that the pain is worse when he coughs. It is diffuse across his whole abdomen and does not radiate to other regions of the body. He denies prior similar episodes. On physical exam, the abdomen feels rigid and there is a positive Blumberg sign. Which of the following is the most likely diagnosis?