Challenging Diagnostic Dilemma: Mesenteric Desmoid Tumor Masquerading as Perforated Peritonitis.

Desmoid fibromatosis is a rare benign neoplasm of the soft tissue. Primary desmoid neoplasms rarely occur in the small bowel and are primarily found in patients with a previous abdominal surgery or irradiation history. They are challenging to diagnose at the time of presentation due to a lower incidence and their non-specific presentation making it difficult to distinguish from other intra-abdominal neoplasms, such as gastrointestinal stromal tumors (GISTs), which may present with similar symptoms. We like to present a case of a 34-year-old male with a four-day history of abdominal pain with worsening severity and one episode of non-bloody vomiting. Physical examination was significant for generalized abdominal tenderness with positive rebound and board-like rigidity. A computed tomography (CT) scan of the abdomen showed the presence of a lower abdominal mass of unknown etiology with free air foci and free intraperitoneal fluid either due to rupture of the suspicious mass or secondary to infection by an air-producing organism. The patient was immediately taken for emergency surgery, the tumor was resected successfully, and a specimen collected was sent for histopathology, which came out to be a desmoid tumor. We aim to highlight the importance of keeping a broad differential diagnosis in a patient with acute abdomen and symptoms of peritonitis.