What is the condition indicated by a dog exhibiting high fever and lethargy with a diagnosis of peritonitis?

The condition indicated by a dog exhibiting high fever and lethargy with a diagnosis of peritonitis is inflammation of the internal abdominal lining. Peritonitis occurs when this lining, known as the peritoneum, becomes inflamed due to infection or irritation, often resulting from the presence of bacteria, foreign materials, or other irritants in the abdominal cavity.

The symptoms of high fever and lethargy are consistent with this condition, as systemic inflammation typically triggers a fever and can lead to significant lethargy due to the body’s response to infection or illness. In addition, the peritoneum's inflammation can result from causes such as gastrointestinal perforations, abscesses, or complications from surgical procedures, all of which lead to serious inflammatory reactions.

The other choices refer to different conditions that do not align with the symptoms presented. Fluid in the pericardial sac relates to pericardial effusion and is associated with heart issues, while inflammation of the pleura pertains to conditions affecting the lungs and chest cavity. A ruptured bladder would lead to urine leakage into the abdomen but is a different underlying issue than generalized peritonitis. Therefore, inflammation of the internal abdominal lining remains the appropriate diagnosis in this scenario.