Differential Diagnosis

Based on the provided clinical history and diagnostic findings, the following differential diagnoses are considered:

• 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 irregular anterior soft tissue deposits with internal air pockets are highly suggestive of colorectal cancer. The patient's symptoms of abdominal pain, changes in bowel habits, and the presence of a palpable mass also support this diagnosis.
• Other Likely diagnoses
  • Diverticulitis: The patient's symptoms of left lower quadrant pain and changes in bowel habits could also be consistent with diverticulitis, especially given the presence of a heterogeneous lesion with calcifications inferior to the left kidney on ultrasound.
  • Intestinal Obstruction: The patient's symptoms of abdominal pain and changes in bowel habits could also be consistent with intestinal obstruction, although the CT scan did not show any evidence of bowel obstruction proximal to the lesion.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Perforated Visculitis: The patient's symptoms of abdominal pain and the presence of a palpable mass could also be consistent with perforated visculitis, which is a life-threatening condition that requires prompt diagnosis and treatment.
  • Intra-abdominal Infection: The patient's symptoms of abdominal pain and changes in bowel habits could also be consistent with an intra-abdominal infection, such as an abscess or peritonitis, which would require prompt antibiotic treatment and possible surgical intervention.
• Rare diagnoses
  • Gastrointestinal Lymphoma: The patient's symptoms of abdominal pain and changes in bowel habits could also be consistent with gastrointestinal lymphoma, although this is a rare diagnosis and would require further evaluation and biopsy to confirm.
  • Carcinoid Tumor: The patient's symptoms of abdominal pain and changes in bowel habits could also be consistent with a carcinoid tumor, although this is a rare diagnosis and would require further evaluation and biopsy to confirm.

It is essential to note that the patient's diagnosis of colorectal cancer is highly likely based on the CT scan findings and clinical presentation. However, it is crucial to consider other possible diagnoses and rule out life-threatening conditions that may require prompt intervention.