What Cause of an Ulcer after Roux-en-Y Bypass Surgery?

This 42-year-old woman lost 100 lb during the year after gastric bypass surgery. Now, she is diagnosed as having an ulcer. What might be the cause of the lesion?

A 42-year-old woman presents to the hospital with epigastric pain of several weeks’ duration. She underwent a Roux-en-Y gastric bypass procedure 1 year ago and subsequently lost 100 lb. She undergoes an upper endoscopy for evaluation which demonstrates an ulcer at the gastro-jejunal junction.

What is the most likely cause of the ulcer?