Irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD) share overlapping GI symptoms but differ fundamentally in pathophysiology, diagnosis, and management. IBS is a functional GI disorder characterized by chronic abdominal pain associated with altered bowel habits WITHOUT identifiable structural or biochemical abnormalities. It is a disorder of gut-brain interaction involving visceral hypersensitivity (lowered pain threshold to luminal distension), altered GI motility, and psychosocial factors. IBD (Crohn disease and ulcerative colitis) is an immune-mediated chronic inflammatory condition with identifiable mucosal damage, tissue destruction, and systemic inflammation. Key distinguishing features: IBS has NO alarm features (no weight loss, no GI bleeding, no nocturnal symptoms, no fever, no anemia, NORMAL inflammatory markers, NORMAL calprotectin); IBD has alarm features (weight loss, bloody stools, nocturnal diarrhea, fever, anemia, elevated CRP/ESR, elevated fecal calprotectin). Fecal calprotectin is the single most useful non-invasive test for distinguishing IBS from IBD: elevated in IBD (reflects neutrophilic intestinal inflammation), NORMAL in IBS. IBS is diagnosed by Rome IV criteria (clinical criteria — no specific diagnostic test), while IBD requires endoscopy with biopsy for definitive diagnosis. IBS prevalence is 10-15%...