A patient with intestinal malabsorption is found to markedly improve when flour products (bread, noodles, etc.) are removed from his diet. At the height of the patient's disease, marked histologic changes would be seen at which of the following sites?

The patient has celiac disease, which is apparently an acquired hypersensitivity to the gluten (such as gliadin) in wheat. Unlike tropical sprue (which may be related to enterotoxigenic E. coli infection), which involves the entire small bowel, celiac sprue is usually limited to the proximal small bowel. This may occur because the gluten antigens have not yet been digested at this point in their journey through the bowel. A gluten-free diet usually restores the small bowel mucosa.