Most Likely Diagnosis: Ulcerative Colitis
The most likely diagnosis is ulcerative colitis (Option A), given the classic triad of abdominal pain, bloody diarrhea, and weight loss over 3 months, combined with a strong family history of identical symptoms and positive fecal occult blood testing. 1, 2
Clinical Reasoning
Why Ulcerative Colitis is Most Likely
The clinical triad of abdominal pain, bloody diarrhea, and weight loss is characteristic of inflammatory bowel disease, particularly UC in younger patients 1, 2
A strong family history of similar symptoms (two relatives with identical complaints) strongly suggests a genetic inflammatory bowel disease rather than other conditions 1
UC has significant genetic predisposition, and family history is a key risk factor for this diagnosis 1
Weight loss correlates with more severe inflammatory bowel disease activity and indicates chronic inflammation rather than functional disorders 1, 2
The 3-month duration represents chronic inflammation consistent with IBD rather than acute infectious or self-limited processes 2
Why NOT Familial Adenomatous Polyposis (Option B)
FAP is unlikely because patients typically present with polyp-related complications rather than the inflammatory symptoms of bloody diarrhea, abdominal pain, and weight loss 1
FAP screening typically begins around age 20 years in at-risk families, and affected relatives would have been diagnosed with hundreds to thousands of polyps, not similar inflammatory symptoms 3
The family history pattern described (relatives with identical symptoms) favors inflammatory bowel disease over a polyposis syndrome 1
Why NOT Diverticulosis (Option C)
Diverticulosis is highly unlikely in a young patient (implied by the clinical context), as it predominantly affects elderly populations 1, 4
Diverticulosis accounts for approximately 30% of large bowel pathology in the elderly, not in younger age groups 4
The strong family history of similar symptoms does not fit the pattern of diverticular disease, which is not typically inherited in this manner 1
Essential Diagnostic Workup
Immediate Laboratory Testing
Complete blood count, ESR, CRP, serum albumin, and fecal calprotectin to assess for anemia, inflammation, and disease activity 1, 2
Stool cultures to exclude infectious causes including C. difficile, which must be ruled out before diagnosing IBD 1, 2
Gold Standard Diagnostic Test
Colonoscopy with multiple biopsies is mandatory and represents the gold standard for UC diagnosis, even if mucosa appears normal 1, 2
Do not delay endoscopic evaluation based on empiric treatment trials; colonoscopy is essential for definitive diagnosis 2
Critical Red Flags Present
Weight loss is an alarm feature that mandates aggressive workup and excludes functional disorders like irritable bowel syndrome 3, 2
Bloody diarrhea indicates inflammatory bowel pathology rather than functional disease 2
Chronic symptoms (3 months duration), continuous diarrhea, and weight loss are suggestive of organic disease requiring endoscopic evaluation 3
Common Pitfalls to Avoid
Never diagnose irritable bowel syndrome in any patient with weight loss, as this is an absolute exclusion criterion for functional disorders 3, 2
Always exclude infectious causes before initiating immunosuppressive therapy for presumed IBD 2
Do not attribute symptoms to functional disorders when alarm features (weight loss, bloody stools) are present 3