The biopsy location is the duodenum. Note the presence of the surface mucosal villi and the Brunner's glands in the base of the mucosa.
The diagnosis is adenocarcinoma. Note the disorganized arrangement of the lining columnar epithelium, the nuclear hyperchromasia and enlargement and the gland within gland pattern. It could not be determined from the biopsy alone, if this neoplasm is originating in the ampulla of Vater, the distal common bile duct, the duodenum proper, or the pancreas with secondary direct extension into the duodenum. That determination is better made by the correlation of the biopsy findings with the endoscopic appearance and radiographic imaging studies.