Background: Primary signet ring cell carcinoma (PSRCC) is a rare, aggressive subtype of colorectal cancer with limited literature coverage that often affects young adults. Its nonspecific presentation and delayed diagnosis contribute to its poor prognosis.
Case Presentation: A 23-year-old male presented with two weeks of abdominal pain, nausea, vomiting, and constipation. CT imaging revealed a small bowel obstruction at the terminal ileum, with an extensive workup showing an elevated carcinoembryonic antigen as the only other abnormality.
Interventions: After failed decompression, further imaging confirmed complete obstruction. The patient underwent laparotomy with right colectomy and ileocolic anastomosis, revealing an obstructing mass.
Outcomes: Pathology showed poorly differentiated adenocarcinoma with signet ring features at the ileocecal valve, showing full penetration into the serosa and involving the appendix.
Conclusion: PSRCC should be considered in young patients with nonspecific abdominal complaints. Although CEA is not routinely used in screening, it was valuable in this case with otherwise negative testing which allowed narrowing of the differential.

Keywords: Primary signet ring cell carcinoma (PSRCC); Colorectal cancer (CRC); Carcinoembryonic antigen (CEA)