A 39-year-old woman complains of developing painful pale fingers on cold exposure for the past 5 years. Recently, she has noticed swollen fingers and tight skin, which limit flexion and extension. She also has new abdominal symptoms that are bothersome. On examination, the skin on the fingers is smooth and shiny with associated edema. The rest of the examination is normal. Which part of the gastrointestinal (GI) tract is most frequently involved in this condition?