What is the single most likely diagnosis in an 87‑year‑old man presenting with abdominal pain and rectal bleeding?

Most Likely Diagnosis: Diverticulitis

In an 87-year-old man presenting with abdominal pain and rectal bleeding, diverticulitis (Option C) is the single most likely diagnosis, accounting for approximately 30% of large bowel pathology in elderly patients and representing one of the most common causes of lower GI bleeding in this age group. 1, 2

Clinical Reasoning

Age-Specific Epidemiology

• Over 60% of patients above age 80 have colonic diverticula, with diverticulitis affecting 10-25% of those with diverticulosis 3, 4
• Diverticular disease prevalence increases from 5% in those under 40 to 65% in those ≥65 years of age 4
• Elderly patients are at higher risk for complicated diverticulitis requiring urgent surgery 5, 4

Presentation Pattern in Elderly

• Only 50% of elderly patients with acute left colonic diverticulitis present with classic left lower quadrant pain, 17% have fever, and 43% lack leukocytosis 5
• The combination of abdominal pain and rectal bleeding in this age group strongly suggests diverticular disease, as bleeding occurs from erosion of the vasa recta at the diverticulum neck 3
• Elderly patients frequently present with atypical symptoms, making clinical diagnosis challenging 6, 5

Why Other Diagnoses Are Less Likely

Colorectal Cancer (Option E)

• While cancer accounts for 60% of large bowel obstructions and must be excluded, it typically presents with chronic symptoms, weight loss, and altered bowel habits rather than acute pain with bleeding 1, 2
• Cancer is a critical "can't-miss" diagnosis but less likely as the single most likely cause of acute presentation 2

Ischemic Colitis (Option I - Acute Mesenteric Ischemia)

• Presents with "pain out of proportion to examination" and bloody diarrhea, but typically lacks the localized tenderness pattern 1, 5
• More commonly associated with cardiovascular risk factors and sudden onset 1

Inflammatory Bowel Disease (Options B, D)

• New-onset IBD in an 87-year-old is uncommon, as only up to 15% of IBD diagnoses occur after age 60 6
• Would typically present with chronic diarrhea, urgency, and systemic symptoms rather than acute pain with bleeding 6

Other Options

• Gastroenteritis (H): Would present with diarrhea as primary symptom, not rectal bleeding with pain 6
• Appendicitis (F): Right-sided pain; complicated appendicitis in elderly has 18-70% perforation rate but wrong location for this presentation 5
• GERD (A): Upper GI symptoms, not rectal bleeding 1
• UTI (G): Dysuria and urinary symptoms, not GI bleeding 6

Immediate Diagnostic Approach Required

Mandatory Imaging

• CT scan with IV contrast is the gold standard, with 98-99% sensitivity and 99-100% specificity for diagnosing diverticulitis and distinguishing complicated from uncomplicated disease 6, 2, 5
• CT is superior to ultrasound in elderly patients (68% vs 48% accurate diagnosis) and critical for detecting perforation, abscess, or ischemia 6

Essential Laboratory Studies

• Complete blood count, serum albumin, ferritin, C-reactive protein 6, 2
• Serum lactate and procalcitonin to assess for bowel ischemia and necrotic damage 2, 5
• Clostridioides difficile testing is mandatory in all elderly patients with diarrhea, regardless of antibiotic history 6, 2

Critical Pitfall to Avoid

• Do not perform colonoscopy during acute presentation, as it increases perforation risk; defer colonoscopy 4-6 weeks after resolution to exclude malignancy 7, 8, 3
• Normal inflammatory markers do not exclude diverticulitis in elderly patients due to atypical presentations 5

Disposition and Management

Hospitalization Criteria

• Elderly patients with diverticulitis have significantly higher mortality (up to 8%) and more frequently require surgery (22%) 5
• Immediate hospital admission is indicated for this 87-year-old with abdominal pain and rectal bleeding pending CT results 1, 5

Treatment Based on CT Findings

• Uncomplicated diverticulitis: May consider outpatient management only if immunocompetent, no sepsis, and adequate social support 8, 3
• Complicated diverticulitis: Requires broad-spectrum antibiotics covering E. coli, Enterobacteriales, and Clostridiales, with percutaneous drainage for accessible abscesses 1, 8
• Perforation with peritonitis: Immediate surgical consultation required 1, 5