SERRATED MISMATCH REPAIR-DEFICIENT ALTERATIONS IN RECTAL MUCOSAL PROLAPSE SYNDROME: A CASE REPORT AND LITERATURE REVIEW

Solitary rectal ulcer syndrome/mucosal prolapse syndrome (SRUS/MPS) is an uncommon, often misdiagnosed benign disorder of the rectum arising from chronic mechanical mucosal trauma. Its co-occurrence with sessile serrated polyps is rare and incompletely characterized, with diagnostic and pathogenetic significance in colorectal medicine. 
We report the case of a 36-year-old woman with a two-year history of rectal bleeding, mucus discharge, tenesmus, and alternating bowel habits that had failed dietary modification, laxatives, and topical steroids. Colonoscopy identified multiple ulcerated polypoid lesions approximately 6 cm from the anal margin, and pelvic magnetic resonance imaging (MRI) demonstrated focal rectal wall thickening with mucosal irregularity, raising concern for malignancy. Following multidisciplinary tumor board discussion, the patient underwent surgical resection. Histology revealed fibromuscular obliteration of the lamina propria characteristic of mucosal prolapse alongside diffusely serrated, saw-toothed crypts with basal dilation and mild adenomatous change considered for sessile serrated polyps. Immunohistochemistry (IHC) demonstrated combined loss of MLH1 and HES1 expression within the serrated crypts with an elevated Ki-67 proliferative index, while other mismatch repair proteins remained intact. At 18-month follow-up, the patient was asymptomatic with no endoscopic recurrence. 
This case illustrates that chronic mucosal prolapse injury can generate serrated, mismatch-repair-deficient morphology mimicking neoplasia, underscoring the need for careful histopathological correlation before presuming malignancy in similarly presenting rectal lesions.