Impression and Differential Diagnosis

Based on the provided history and physical examination, the patient presents with recurring left lower quadrant (LLQ) pain, changes in bowel habits, and a bulging mass in the upper left abdomen. The diagnostic findings, including ultrasound and CT scan results, suggest a significant abdominal pathology.

Differential Diagnoses

• 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 possible serosal/peritoneal invasion are highly suggestive of colorectal cancer. The patient's symptoms of LLQ pain, changes in bowel habits, and the presence of a palpable mass further support this diagnosis.
• Other Likely Diagnoses:
  • Diverticulitis: Although the CT scan did not specifically mention diverticula, the patient's age and symptoms could also be consistent with diverticulitis, especially if the mass-like thickening is inflammatory rather than neoplastic.
  • Intestinal Obstruction: The severe luminal narrowing could potentially lead to or be a sign of intestinal obstruction, which would be a complication of the primary pathology.
• Do Not Miss Diagnoses:
  • Perforated Visculitis or Vasculopathy: Conditions like polyarteritis nodosa or other vasculitides could lead to bowel ischemia or infarction, presenting with similar symptoms and would be critical to diagnose early due to their high morbidity and mortality.
  • Intra-abdominal Infections (e.g., Abscess): Although the patient does not have fever or significant leukocytosis mentioned, an intra-abdominal abscess could present with localized pain and mass, and would require urgent drainage.
• Rare Diagnoses:
  • Gastrointestinal Lymphoma: Although less common, lymphoma could present with similar symptoms and findings, especially if there's significant lymphadenopathy or systemic symptoms, which are not prominently featured in this case.
  • Mesenteric Cyst or Other Rare Tumors: The initial ultrasound suggestion of an infrarenal mass could also be considered in the differential, although the CT findings lean more towards a colonic origin.

Diagnostic Plan

1. Colonoscopy: To directly visualize the colon and obtain a biopsy of the mass for histopathological examination.
2. Complete Blood Count (CBC) and Blood Chemistry: To assess for signs of infection, inflammation, or metabolic disturbances.
3. Tumor Markers (e.g., CEA): If colorectal cancer is highly suspected, to aid in diagnosis and potentially in monitoring response to treatment.
4. Staging CT Scan or MRI: If colorectal cancer is confirmed, to assess for distant metastases and local extent of disease.
5. Surgical Consultation: Given the potential need for resection of the tumor and possibly affected bowel segments, as well as the management of any complications like obstruction or perforation.