A 24-year-old male presents with a 6-month history of recurrent bloody diarrhea, lower abdominal cramping, and tenesmus. Colonoscopy reveals continuous, erythematous, and friable mucosa starting at the rectum and extending proximally to the splenic flexure. The inflammation is confined to the mucosal and submucosal layers. Which of the following is the most likely diagnosis?
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A
Crohn's disease
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B
Irritable bowel syndrome
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C
Ulcerative colitis
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D
Celiac disease