A 19-year-old man comes to student health because his clothes no longer fit — he has lost 12 pounds over six months without dieting. His stools have become pale, greasy, and foul-smelling, often floating in the toilet. Over the past three months he has noticed easy bruising and bleeding gums when he brushes his teeth. He has no chronic illnesses and no prior bowel surgery. Physical examination reveals ecchymoses scattered across both lower extremities. Laboratory studies show a prolonged PT/INR; serum vitamins A, D, and K are all below the reference range. Serum B12 is 410 pg/mL (normal). Serum folate is normal. Stool fat testing is markedly positive. Following an oral dose of D-xylose, urinary D-xylose excretion at 5 hours is markedly reduced.
Damage to which of the following intestinal sites is most likely responsible for this patient's nutrient deficiency pattern?