A Presentation of Meckel’s Diverticulum With Malignancy

Introduction: Meckel’s diverticulum (MD) is the most common congenital gastrointestinal anomaly resulting from incomplete obliteration of the omphalomesenteric duct. Although often asymptomatic, MD may rarely harbor malignancies. The management of incidentally discovered MD in adults remains controversial. Case Presentation: A man in his 70s presented with abdominal pain and iron deficiency anemia. Computed tomography demonstrated a 13 × 14 cm heterogeneously enhancing right lower quadrant mass concerning for a primary mesenteric neoplasm. The patient underwent surgical resection, and intraoperative findings revealed a large mass arising from a previously undiagnosed MD. En bloc segmental small bowel resection with associated mesentery was performed. Postoperative recovery was notable for transient ileus. Final pathology demonstrated high-grade undifferentiated carcinoma. Molecular profiling revealed high microsatellite instability (MSI-H) status, programmed death-ligand 1 (PD-L1) positivity, high tumor mutational burden, and an ATM mutation, guiding initiation of immunotherapy. Discussion: Neoplasms in MD are rare and most commonly neuroendocrine in origin; undifferentiated carcinoma is exceedingly uncommon. This case highlights the diagnostic challenge of MD presenting as a large mesenteric mass and underscores the importance of molecular profiling in identifying actionable biomarkers, particularly in rare malignancies lacking established treatment guidelines. Contemporary evidence supports a selective approach to incidental MD resection, with risk stratification based on factors such as age, sex, and diverticulum size. Conclusion: Undifferentiated carcinoma should be considered in the differential diagnosis of large mesenteric masses. This case emphasizes the importance of recognizing atypical presentations of MD and the critical role of molecular characterization in guiding personalized therapy when conventional management strategies are limited.