Congo Red Unmasks the Culprit Behind Gastrointestinal Bleeding.

Immunoglobulin light chain (AL) amyloidosis arises from a plasma cell dyscrasia and is characterized by extracellular deposition of misfolded light chains. Gastrointestinal (GI) involvement is uncommon but clinically important because amyloid-related mucosal ischemia and vascular fragility can precipitate significant bleeding. A 65-year-old woman with systemic AL (lambda) amyloidosis with cardiac and renal involvement and stage II IgA lambda multiple myeloma presented with several hours of acute hematochezia. Her hemoglobin was 9.6 g/dL on presentation. Computed tomography angiography showed no active contrast extravasation, with mild-to-moderate distal rectal wall thickening and increased intraluminal fluid in the left colon. Colonoscopy on hospital day three demonstrated an adherent sigmoid clot (22-40 cm from the anal verge) without active bleeding; the examination was limited by clot burden. Four days later, she developed recurrent large-volume hematochezia with hemoglobin nadir of 7.9 g/dL. Repeat colonoscopy on hospital day seven demonstrated sigmoid ulceration at the prior clot site, and biopsies were obtained. Routine histopathology showed ulcerated granulation tissue consistent with ischemic-type mucosal injury. Congo red staining was focally positive for amyloid deposition, confirming colonic amyloid involvement. She was discharged without recurrent hematochezia. This case underscores the diagnostic value of requesting amyloid evaluation (including Congo red staining) when endoscopic findings and routine histology are non-specific, particularly in patients with known or suspected plasma cell dyscrasia and recurrent or unexplained GI bleeding.