A 55-year-old woman presents with chronic diarrhea and is found to have a normal colonoscopy but colonic biopsy reveals thickened subepithelial collagen band > 10 µm. What is the diagnosis?
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A
Collagenous colitis (microscopic colitis)
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B
Ulcerative colitis
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C
Crohn's colitis
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D
Ischemic colitis