Review Question - QID 109576

A 28-year-old woman presents with right lower quadrant abdominal pain, fatigue, and intermittent diarrhea for the past 9 months. She also reports weight loss and believes it to be due to a decreased appetite as she has been a vegetarian for the past year. She has noticed herself being more "forgetful" and she denies seeing any blood in her stool, changes in diet, infection, or recent travel history. Her temperature is 99.5°F (37.5°C), blood pressure is 112/72 mmHg, pulse is 89/min, and respirations are 17/min. Physical examination is unremarkable. Laboratory testing is shown below:

Hemoglobin: 10.8 g/dL
Hematocrit: 32%
Platelet count: 380,000/mm^3
Mean corpuscular volume: 118 µm^3
Reticulocyte count: 0.27%
Leukocyte count: 9,900 cells/mm^3 with normal differential
Erythrocyte sedimentation rate: 65 mm/h

A colonoscopy is performed and demonstrates focal ulcerations with polypoid mucosal changes adjacent to normal appearing mucosa. A biopsy is obtained and shows ulcerations and acute and chronic inflammatory changes. Involvement of which of the following sites most likely explains this patient's clinical presentation?