Episode two of the Chronic Diarrhea and Malabsorption chapter works the patient who arrives after a clean standard workup. Three entities account for most of what the first pass missed: bile acid malabsorption, microscopic colitis, and a structured algorithm for obscure chronic diarrhea. Board-tested reasoning on typing, testing, and the trials that are diagnostic and therapeutic in one step. Topics covered Bile acid malabsorption Camilleri classification Hundred-centimeter ileal resection rule Post-cholecystectomy diarrhea Microscopic colitis Lymphocytic vs collagenous colitis Obscure chronic diarrhea algorithm IBS-D overlap Key decisions Under 100 cm of resected ileum, cholestyramine fixes bile acid diarrhea; over 100 cm the picture flips to steatorrhea and cholestyramine worsens it, so use medium-chain triglycerides plus a low-fat diet. Idiopathic (type 2) bile acid diarrhea hides inside the IBS-D label; low fasting FGF19 and elevated C4 are the serum surrogates, and an empiric sequestrant trial responding within a week or two confirms it in the US where the SeHCAT scan is unavailable. Microscopic colitis is diagnosed only on random biopsies from at least the right and left colon; lymphocytic colitis needs more than 20 intraepithelial lymphocytes per 100 surface cells, collagenous colitis a subepithelial band over 10 microns, both with preserved crypt architecture. First therapeutic move in microscopic colitis is removing the offending drug (NSAIDs, PPIs especially lansoprazole, SSRIs) and smoking cessation; budesonide six to eight weeks is first-line for moderate to severe disease. A celiac patient with persistent watery diarrhea on a strict gluten-free diet needs colonoscopy with random biopsies, not a repeat duodenal biopsy, because microscopic colitis is far more common in celiac. Obscure diarrhea is worked in prevalence order: sequestrant, then rifaximin, then lactose-free, then low-FODMAP, then pancreatic enzymes, with HLA testing to exclude celiac before committing to lifelong gluten avoidance. For the full chapter with MCQs, tables, and primary-guideline references, visit www.boardpearls.com. Questions or feedback: [email protected]. (00:00) - Three commonly missed causes after a clean workup (00:38) - Bile acid malabsorption: mechanism and phenotype (01:26) - Camilleri types and the hundred-centimeter rule (03:32) - Diagnostic testing: SeHCAT, C4, FGF19, empiric trial (05:08) - Microscopic colitis: the biopsy-only diagnosis (06:36) - Random biopsies, histology, and severity-based treatment (08:17) - The obscure chronic diarrhea algorithm (09:19) - Sequential empiric trials and the IBS-D endpoint

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“What did the council decide on item 42?” — answered with the speaker and the moment.

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