Irritable bowel syndrome (IBS)

Crohn disease is a chronic transmural inflammatory bowel disease that can affect any segment from mouth to anus in skip lesions, most classically the terminal ileum, and is confirmed by ileocolonoscopy showing cobblestoning, skip areas, and non-caseating granulomas on biopsy. Management follows an ordered sequence of glucocorticoid or budesonide induction followed by immunomodulator or biologic maintenance, with surgery reserved for fixed complications rather than cure.

Ulcerative colitis is a chronic immune-mediated disease of continuous mucosal inflammation that begins in the rectum and extends proximally through the colon, presenting with bloody diarrhea and tenesmus and confirmed by colonoscopy with biopsy. Treatment is stepped by extent and severity from 5-aminosalicylates through corticosteroids, immunomodulators, and biologics, with acute severe disease requiring IV corticosteroids and medical or surgical rescue, and colectomy remains curative.

Celiac disease is an immune-mediated small-intestinal enteropathy triggered by dietary gluten in genetically predisposed individuals, presenting with malabsorptive diarrhea, iron-deficiency anemia, or dermatitis herpetiformis. Diagnosis requires IgA tissue transglutaminase serology with a total IgA level while the patient still eats gluten, confirmed by duodenal biopsy showing villous atrophy, and the only treatment is a strict lifelong gluten-free diet.

Microscopic colitis is a cause of chronic watery, non-bloody diarrhea in older women with a grossly normal colonoscopy, diagnosed only by random colon biopsy showing lymphocytic or collagenous histology. Management starts with stopping culprit drugs and antidiarrheals, then oral budesonide for induction and, if needed, maintenance.

Lactose intolerance is osmotic diarrhea, bloating, and flatulence caused by deficiency of the brush-border enzyme lactase, most often the primary age-related decline seen in people of Asian, African, or Native American descent. Diagnosis rests on symptom resolution with lactose elimination and recurrence on rechallenge, confirmed when needed by a hydrogen breath test, and treatment is dietary lactose restriction with lactase enzyme replacement and calcium and vitamin D intake.

Small intestinal bacterial overgrowth is excessive bacterial colonization of the proximal small bowel, driven by stasis from strictures, blind loops, or dysmotility, or by loss of gastric-acid and immune defenses, producing bloating, diarrhea, and fat and B12 malabsorption. Diagnosis rests on a carbohydrate breath test or jejunal aspirate culture, and treatment corrects the underlying cause while a course of rifaximin or another antibiotic reduces the bacterial burden.

Colorectal cancer is an adenocarcinoma that arises through the adenoma-carcinoma sequence and presents differently by location, with right-sided tumors causing occult bleeding and iron-deficiency anemia and left-sided tumors causing obstruction and hematochezia. Colonoscopy with biopsy confirms the diagnosis, TNM staging on CT chest/abdomen/pelvis with CEA guides treatment, and management ranges from surgical resection alone for localized disease to adjuvant FOLFOX for node-positive colon cancer, neoadjuvant chemoradiation for rectal cancer, and molecularly targeted systemic therapy for metastatic disease.

Giardiasis is infection of the proximal small bowel with the protozoan Giardia lamblia (duodenalis), acquired from cysts in contaminated fresh water or food, causing malabsorptive, foul-smelling, greasy diarrhea with bloating and weight loss over weeks. Diagnosis rests on a stool antigen or nucleic acid test, and treatment is a short course of tinidazole, metronidazole, or nitazoxanide.

Fecal incontinence is the recurrent, involuntary passage of fecal material for at least three months, most often from a combination of obstetric or surgical sphincter injury, altered stool consistency, and impaired rectal sensation. Diagnosis rests on history and digital rectal exam followed by anorectal manometry and anal endosonography, and treatment moves stepwise from correcting stool consistency and reversible causes to pelvic floor therapy, biofeedback, and, for refractory or structural disease, bulking agents, sacral nerve stimulation, or sphincteroplasty.