A 94-year-old female nursing home resident is referred for evaluation of anemia of 8 g/dL. She has dementia, and adequate documentation of her past medical history is not available. She eats well and is cooperative. Examination reveals evidence of cognitive impairment, primitive reflexes, and a well-healed midline abdominal scar. Her blood film is shown in Figure below. You presume a relationship between the anemia and the previous surgery. Which of the following surgical procedures is most likely to cause her current anemia?