Small Intestine, Colon, and Anorectum
Core concepts
Build a framework around absorption and obstruction. Celiac disease is a small-bowel immune enteropathy: positive tissue transglutaminase IgA with total IgA checked, duodenal biopsy showing villous atrophy, and a strict gluten-free diet. Tropical sprue, bacterial overgrowth, and Whipple disease (Tropheryma whipplei, PAS-positive macrophages) are key differentials. Short bowel syndrome management depends on which segment is resected: ileal resection causes bile acid diarrhea and vitamin B12 deficiency, while extensive resection risks fat-soluble vitamin loss.
For the colon, know diverticular disease complications (diverticulitis, abscess, fistula, stricture) and when to image. Ischemic colitis typically affects watershed regions and presents with pain out of proportion followed by bloody diarrhea. Anorectal disease questions test hemorrhoids, fissures (posterior midline), and abscess versus fistula. Fecal incontinence evaluation begins with history and exam, then anorectal manometry and endoanal ultrasound.
Traps
Celiac serology can be falsely negative in IgA deficiency, so always check total IgA. Questions confuse iron deficiency location: duodenal and proximal jejunal disease causes iron deficiency, ileal disease causes B12 deficiency. Do not attribute rectal bleeding in older adults to hemorrhoids without considering colorectal cancer.
Rule of thumb
For suspected celiac disease, keep the patient on gluten until testing is complete; a negative serology in a patient already gluten-free is uninterpretable.