<![CDATA[Introduction. Low-grade appendiceal mucinous neoplasm (LAMN) is a rare mucin-producing epithelial tumor identified in roughly 0.13–1% of appendectomies, with a recognized risk of pseudomyxoma peritonei (PMP) on rupture. Synchronous coexistence with perforated ascending colon diverticulitis and an ipsilateral ovarian mature cystic teratoma is exceptional and creates a triple-layered diagnostic and operative challenge. Case Presentation. A 52-year-old Indonesian female presented with one month of right lower quadrant (RLQ) pain that worsened over forty-eight hours, accompanied by nausea and vomiting. Examination revealed McBurney tenderness, local muscular defense, and a palpable RLQ mass. Leukocytosis with neutrophilic predominance (white blood cells 19.18×10³/µL; neutrophils 91%) was noted. Ultrasonography demonstrated a lobulated, hyperechoic pelvic mass measuring 10.46 cm. Contrast-enhanced computed tomography showed a solid–cystic pelvic lesion with fat and calcification consistent with mature cystic teratoma alongside a periappendicular infiltrate. Exploratory laparotomy disclosed ~50 mL of purulent fluid, an abnormally enlarged appendix, multifocal ascending colon diverticulitis with perforation, dense adhesions, and a right ovarian mass. Right hemicolectomy with transverse colon stump and end ileostomy together with total abdominal hysterectomy and bilateral salpingo-oophorectomy (TAH-BSO) were performed. Histopathology confirmed LAMN with periappendicular infiltrate and a mature cystic teratoma of the right ovary. Conclusion. Coexistence of LAMN, perforated colonic diverticulitis, and mature ovarian teratoma can mimic adnexal or inflammatory disease and obscure recognition of the appendiceal neoplasm. Vigilant intra-operative inspection of the appendix and adjacent colon, oncologically appropriate resection, and definitive histopathology are essential to prevent PMP.]]>
