Top 15 Differential Diagnoses

Likelihood-Ranked Differential

🔴 HIGH PRIORITY (Must Rule Out First)

• Why: Affects young women disproportionately. Crohn's causes transmural inflammation → bloody/mucoid stools, iron-deficiency anemia (chronic blood loss + malabsorption), and significant bloating from dysmotility and bacterial overgrowth.
• Typical: RLQ pain, skip lesions, perianal disease, weight loss
• Atypical: May present primarily as anemia or fatigue with minimal overt GI bleeding
• Key workup: Colonoscopy with biopsy, CRP/ESR, fecal calprotectin, CTE/MRE
• — Harrison's Principles of Internal Medicine 22E; Yamada's Textbook of Gastroenterology

• Why: Peak incidence in young adults. UC causes mucosal inflammation of the colon → hematochezia, crampy abdominal pain, bloating, and chronic anemia from continuous colonic blood loss.
• Typical: Bloody diarrhea, tenesmus, urgency, continuous colonic involvement from rectum
• Atypical: Constipation-predominant variant; extraintestinal manifestations (arthritis, uveitis, skin lesions) preceding GI symptoms
• Key workup: Colonoscopy, biopsy, stool cultures to exclude infection
• — Goldman-Cecil Medicine; Harrison's Principles of Internal Medicine 22E

• Why: Strongly female-predominant in young adults. Classic triad includes iron-deficiency anemia (duodenal malabsorption), bloating/distension (fermentation from malabsorption), and diarrhea which can occasionally be bloody in severe cases.
• Typical: Steatorrhea, weight loss, fatigue, aphthous ulcers
• Atypical: Silent celiac with anemia as sole presentation; dermatitis herpetiformis
• Key workup: Anti-tTG IgA, total IgA, HLA-DQ2/DQ8; duodenal biopsy (gold standard)
• — Robbins & Kumar Basic Pathology; Yamada's Textbook of Gastroenterology

• Why: Although uncommon under 30, Lynch syndrome (hereditary non-polyposis CRC) and familial adenomatous polyposis must be considered. Polyps and early CRC cause occult/frank bleeding, iron-deficiency anemia, and bloating.
• Typical: Hematochezia, change in bowel habits, family history
• Atypical: Isolated iron-deficiency anemia without obvious rectal bleeding; young patient without recognized family history
• Key workup: Colonoscopy, genetic testing (MSH2/MLH1/PMS2), CEA
• — Tintinalli's Emergency Medicine; Goldman-Cecil Medicine

• Why: The most common cause of bright red rectal bleeding overall. Internal hemorrhoids cause painless hematochezia; chronic blood loss leads to iron-deficiency anemia; straining increases intraabdominal pressure → bloating sensation.
• Typical: Bright red blood on paper/in bowl, sensation of incomplete evacuation
• Atypical: Anemia as presenting finding without the patient noticing bleeding; prolapsed hemorrhoids with mucus discharge
• Key workup: Anoscopy, proctoscopy; colonoscopy to exclude proximal pathology

🟠 MODERATE PRIORITY

• Why: IBS-C/IBS-M is 2–3× more common in young women. Bloating is the cardinal symptom. Constipation can cause anorectal trauma → small volume rectal bleeding. Anemia is atypical but can occur from concurrent deficiencies.
• Typical: Bloating, alternating bowel habits, symptoms relieved by defecation, no alarm features
• Atypical: Overlapping with SIBO; bleeding from hemorrhoids secondary to straining
• Key workup: Rome IV criteria (diagnosis of exclusion); CBC, CRP, celiac serology, colonoscopy to exclude organic disease
• — Yamada's Textbook of Gastroenterology; Harrison's Principles

• Why: Young woman, non-pregnant — endometriosis is a leading diagnosis. Endometrial implants on the rectosigmoid cause cyclic or chronic hematochezia, bloating (particularly perimenstrual), and anemia from menstrual + GI blood loss.
• Typical: Cyclic rectal bleeding coordinated with menses, dysmenorrhea, dyspareunia, pelvic pain
• Atypical: Bowel symptoms dominate without obvious gynecologic complaints ("silent" intestinal endometriosis); diagnosed incidentally on colonoscopy or laparoscopy
• Key workup: Pelvic ultrasound, MRI pelvis, laparoscopy (gold standard), CA-125 (nonspecific)

• Why: SIBO causes fermentation of carbohydrates → bloating/distension. Bacterial competition for nutrients causes B12 and iron deficiency (anemia). Mucosal irritation can occasionally produce blood-streaked stools.
• Typical: Bloating worse after meals/carbohydrates, chronic diarrhea, flatulence
• Atypical: Constipation-predominant SIBO; can masquerade as IBS
• Key workup: Hydrogen/methane breath test; empirical antibiotic trial (rifaximin)
• — Yamada's Textbook of Gastroenterology

• Why: Tintinalli's specifically flags: "Consider ovarian carcinoma in women with new concerning symptoms of obstruction or bloating." A pelvic mass can compress bowel → constipation with rectal bleeding and bloating; mass effect causes anemia if malignant.
• Typical: Progressive abdominal distension, pelvic fullness, early satiety
• Atypical: Initial presentation as bowel symptoms mimicking IBS; ascites
• Key workup: Pelvic ultrasound, CA-125, CT abdomen/pelvis
• — Tintinalli's Emergency Medicine

• Why: All cause bloody diarrhea, cramping, and bloating. Chronic or recurrent infections cause iron-deficiency anemia. Amoebiasis is particularly relevant if there is travel history.
• Typical: Acute onset, fever, tenesmus, exposure history
• Atypical: Chronic/recurrent course mimicking IBD; C. difficile without recent antibiotic use
• Key workup: Stool cultures, ova and parasites, C. difficile PCR/toxin, colonoscopy in chronic cases

🟡 LOWER PRIORITY (But Clinically Important)

• Why: Uncommon in young women but can occur with OCP use (thrombophilia), vasculitis, or hypercoagulable states. Causes segmental colitis → bloody stools, abdominal pain, bloating.
• Typical: Acute cramping pain followed by bloody diarrhea, usually at watershed zones (splenic flexure)
• Atypical: Subacute presentation; associated with OCP or sickle cell disease
• Key workup: CT abdomen with contrast, colonoscopy (after acute phase), hypercoagulability screen

• Why: Most symptomatic cases present before age 30. Ectopic gastric mucosa causes ulceration → painless rectal bleeding (classically brick-red) and iron-deficiency anemia. Diverticulum causes bloating/intermittent obstruction.
• Typical: Painless rectal bleeding in young adult, rule of 2s (2 inches long, 2 feet from ileocecal valve, 2% population)
• Atypical: Mimics appendicitis; chronic occult bleeding with anemia only
• Key workup: Technetium-99m pertechnetate scan (Meckel's scan)

• Why: AVMs of the GI tract cause chronic occult or frank GI bleeding → anemia. Can produce bloating from associated dysmotility. Rare in young women but can be associated with connective tissue disorders (Ehlers-Danlos, hereditary hemorrhagic telangiectasia).
• Typical: Recurrent GI bleeding without obvious source on routine workup
• Atypical: Associated with aortic stenosis (Heyde's syndrome) or vWD type 2A
• Key workup: Colonoscopy, push enteroscopy, capsule endoscopy, CT angiography

• Why: Female predominance; often presents with watery or mucoid diarrhea, bloating, and cramping. Rarely causes frank hematochezia but can cause occult bleeding and subsequent anemia. Often triggered by NSAIDs, PPIs, or SSRIs.
• Typical: Chronic watery non-bloody diarrhea in middle-aged women (can occur earlier)
• Atypical: Younger patients; blood-streaked stools; associated autoimmune conditions
• Key workup: Colonoscopy with biopsy (normal gross appearance; diagnosis is histological)
• — Robbins & Kumar Basic Pathology

• Why: Autosomal dominant; multiple mucocutaneous and visceral telangiectasias → recurrent GI bleeding → iron-deficiency anemia. GI involvement causes bloating and intermittent hematochezia.
• Typical: Recurrent epistaxis, telangiectasias on lips/tongue/fingers, family history
• Atypical: GI bleeding precedes cutaneous signs; diagnosed late due to low clinical suspicion
• Key workup: Curaçao criteria (epistaxis, telangiectasias, visceral involvement, family history); genetic testing (ENG, ACVRL1/ALK1); capsule endoscopy

Summary Table

Essential Initial Workup for This Patient

Clinical Pearl: In a 26-year-old woman, the combination of all three symptoms (bloating + anemia + bloody stools) makes IBD (Crohn's or UC) and celiac disease the leading diagnoses requiring urgent evaluation. Intestinal endometriosis is a critical diagnosis not to miss, as it is commonly overlooked when GI symptoms dominate the clinical picture.