Differential Diagnosis for a 48-year-old Male with Chronic Bloody Diarrhea and New Onset of Left Lower Quadrant Abdominal Pain

• Single Most Likely Diagnosis
  • Inflammatory Bowel Disease (IBD) - Ulcerative Colitis or Crohn's Disease: Chronic bloody diarrhea is a hallmark symptom of IBD, particularly ulcerative colitis. The new onset of left lower quadrant abdominal pain could indicate a complication such as a flare-up or possible development of a toxic megacolon in ulcerative colitis, or involvement of the sigmoid colon in Crohn's disease.
• Other Likely Diagnoses
  • Diverticulitis: Given the acute onset of left lower quadrant pain, diverticulitis is a strong consideration, especially if the patient has a history of diverticulosis. Bloody diarrhea can sometimes be associated with diverticulitis, although it's more commonly seen with diverticular bleeding.
  • Ischemic Colitis: This condition can cause bloody diarrhea and abdominal pain, particularly in the left lower quadrant if the sigmoid colon is involved. It's more common in older adults and those with risk factors for vascular disease.
• Do Not Miss Diagnoses
  • Colorectal Cancer: Although less likely given the acute onset of symptoms, colorectal cancer can cause chronic bloody diarrhea and abdominal pain. It's crucial to rule out cancer, especially in a patient of this age group, due to its significant implications on treatment and prognosis.
  • Infectious Colitis (e.g., Clostridioides difficile, Salmonella, Shigella): These infections can cause bloody diarrhea and abdominal pain. Identifying and treating the underlying cause is critical to prevent complications and transmission.
  • Mesenteric Ischemia: This is a medical emergency that can present with abdominal pain out of proportion to physical examination findings. While less common, it's a critical diagnosis not to miss due to its high mortality rate if not promptly treated.
• Rare Diagnoses
  • Intussusception: Although more common in children, intussusception can occur in adults and cause abdominal pain and bloody diarrhea. It's often associated with a lead point such as a tumor.
  • Eosinophilic Colitis: A rare condition characterized by eosinophilic infiltration of the colon, which can cause chronic diarrhea, abdominal pain, and sometimes blood in the stool.
  • Microscopic Colitis: Includes lymphocytic and collagenous colitis, which can cause chronic watery diarrhea but less commonly bloody diarrhea. Abdominal pain may be present but is not the dominant symptom.