Differential Diagnosis for 71-year-old Man with Severe Intermittent Lower Abdominal Pain and Rectal Bleeding

• Single most likely diagnosis
  • Diverticulitis: Despite the lack of bowel movements and no change in symptoms after one week, diverticulitis remains a strong consideration given the initial suspicion and the presentation of severe intermittent lower abdominal pain, bright red rectal bleeding, and mucous. The absence of findings on abdominal CT scans does not entirely rule out diverticulitis, especially if the disease is limited to the distal sigmoid colon or if the scans were performed early in the course of the disease.
• Other Likely diagnoses
  • Inflammatory Bowel Disease (IBD): The presence of rectal bleeding, mucous, and abdominal pain could suggest IBD, such as ulcerative colitis or Crohn's disease, especially if diverticulitis is not confirmed.
  • Ischemic Colitis: This condition can present with sudden onset of abdominal pain and rectal bleeding, and while it's more common in older adults, the lack of fever and vomiting doesn't rule it out.
  • Infectious Colitis: Given the positive test for E. Coli and the initial treatment with antibiotics, infectious colitis is a consideration, although the lack of response to antibiotics and the absence of fever make it less likely.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Colorectal Cancer: Any case of rectal bleeding, especially in an older adult, warrants consideration of colorectal cancer. The presence of mucous and the severity of symptoms increase the urgency for ruling out this diagnosis.
  • Mesenteric Ischemia: Although less likely given the lack of vomiting and the specific pattern of pain, mesenteric ischemia is a life-threatening condition that requires prompt diagnosis and treatment.
  • Sigmoid Volvulus: This condition can cause severe abdominal pain and obstruction, leading to ischemia if not promptly treated. The absence of bowel movements could suggest an obstruction.
• Rare diagnoses
  • Intussusception: A rare cause of intestinal obstruction in adults, which can present with abdominal pain, rectal bleeding, and mucous.
  • Solitary Rectal Ulcer Syndrome: This condition can cause rectal bleeding and mucous, but it's less likely given the severity of the abdominal pain and the presence of E. Coli.
  • Eosinophilic Colitis: A rare inflammatory condition that could present with similar symptoms, but it's less common and would require specific diagnostic testing to confirm.