Differential Diagnosis

The patient's presentation of recurrent left lower quadrant pain, changes in bowel habits, and the findings on imaging studies suggest a range of possible diagnoses. Here is a categorized differential diagnosis:

• Single Most Likely Diagnosis

  • Colorectal Cancer: The CT scan findings of a bulky, necrotic, circumferential mass-like thickening in the distal transverse colon with severe luminal narrowing and signs suggestive of serosal/peritoneal invasion are highly suggestive of colorectal cancer. The patient's symptoms of changes in bowel habits and recurrent abdominal pain also support this diagnosis.

• Other Likely Diagnoses

  • Diverticulitis: Although the CT scan did not show a clear diverticulum, the presence of a mass in the left lower quadrant and changes in bowel habits could also suggest diverticulitis, especially if the patient has a history of diverticular disease.
  • Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis could present with similar symptoms and findings, including changes in bowel habits and abdominal pain. However, the absence of other typical symptoms like diarrhea, weight loss, and blood in stool makes this less likely.
  • Intestinal Obstruction: The severe luminal narrowing seen on the CT scan could potentially lead to or be a sign of intestinal obstruction, although the presence of retrograde passage of contrast suggests that the obstruction is not complete.

• Do Not Miss Diagnoses

  • Perforated Visculitis or Vasculopathy: Conditions that affect the blood vessels supplying the intestines, such as vasculitis or thrombosis, could lead to bowel ischemia or infarction and present with severe abdominal pain and changes in bowel habits. These conditions are less common but can be life-threatening if not promptly diagnosed and treated.
  • Intra-abdominal Infections: Although the patient does not have fever or other signs of infection, intra-abdominal infections like abscesses or peritonitis could present with abdominal pain and changes in bowel habits. These conditions require urgent diagnosis and treatment.

• Rare Diagnoses

  • Gastrointestinal Lymphoma: This is a rare condition that could present with abdominal pain, changes in bowel habits, and a mass on imaging studies. However, it is less common than other diagnoses like colorectal cancer.
  • Carcinoid Tumor: A rare type of tumor that could arise in the gastrointestinal tract and present with similar symptoms, although it is much less common than colorectal cancer.
  • Intussusception: A condition where a part of the intestine telescopes into another part, which could cause abdominal pain and changes in bowel habits. It is rare in adults and usually associated with a lead point like a tumor.