A Case Report of Metastatic Melanoma of Unknown Primary with Massive Jejunal Involvement Mimicking Intestinal Lymphoma in a Young Adult: Diagnostic Pitfalls and Surgical Challenges.

Background and Clinical Significance: Malignant melanoma with primary or metastatic intestinal involvement is a rare entity, often diagnosed late and associated with severe complications such as bowel obstruction and perforation. Differential diagnosis of primary intestinal lymphoma may be challenging in the absence of an identifiable primary lesion. Case Presentation: We report the case of a 35-year-old male with no significant medical history who was admitted for persistent abdominal symptoms. Contrast-enhanced abdominal CT revealed a giant circumferential jejunal mass (109/147/156 mm) causing marked luminal stenosis and mesenteric lymphadenopathy, initially raising suspicion of primary intestinal lymphoma. The patient subsequently developed upper intestinal obstruction and severe anemia (Hb 5.5 g/dL), requiring an emergency exploratory laparotomy. Intraoperatively, a voluminous unresectable tumor extending to the mesenteric root was identified, and a feeding jejunostomy was performed. The postoperative course was complicated by tumor perforation and generalized peritonitis, necessitating reoperation. Histopathological examination established the diagnosis of malignant melanoma, with no identifiable primary site, which is most consistent with metastatic melanoma (MUP). PET-CT staging demonstrated metastatic disease (mesenteric, retroperitoneal and supraclavicular lymph nodes, as well as subcutaneous nodules), consistent with a stage IV disease. Molecular analysis revealed a BRAF V600E mutation. Combined immunotherapy (Nivolumab + Ipilimumab) was initiated, resulting in a partial radiological response after three cycles. Conclusions: Intestinal involvement by malignant melanoma might mimic other gastrointestinal malignancies and be the cause of a delayed diagnosis and severe surgical complications. Multidisciplinary management is essential, and modern immunotherapy offers promising outcomes even in advanced-stage disease.