An Unusual Variant of Adenocarcinoma of the Left Colon Associated With Microsatellite Instability: A Case Report.

We report a case of colonic adenosquamous carcinoma with MSI-H (microsatellite instability-high) in a 43-year-old male who presented with bowel obstruction due to a circumferential mass involving the descending colon and splenic flexure. Microscopically, it showed poorly differentiated adenocarcinoma with squamous differentiation, tumor infiltrating lymphocytes >3/high-power field, and mild peritumoral lymphocytic response. Immunohistochemistry was equivocal for MLH-1, PMS-2, and MSH-2, with retention of MSH-6 expression.