A 72-year-old woman with a history of diverticulosis presents with acute onset of left lower quadrant pain, abdominal distension, and absolute constipation for 2 days. She is afebrile (37.2°C), blood pressure 115/75 mmHg, and has a palpable mass in the left lower abdomen. Abdominal examination reveals mild tenderness without guarding. Abdominal X-ray shows a dilated sigmoid colon with a transition point at the rectosigmoid junction, and the proximal colon is also dilated. CT abdomen with contrast shows a narrowed segment at the rectosigmoid with shouldering, and no free air. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Colorectal Cancer (Rectosigmoid) ### Key Clinical Features **Key Point:** The combination of age >70, acute presentation with obstruction, a palpable mass, and CT findings of a narrowed segment with "shouldering" (asymmetric narrowing with irregular margins) is highly suspicious for malignancy. ### Distinguishing Features of Colorectal Cancer Obstruction | Feature | Colorectal Cancer | Sigmoid Volvulus | Diverticulitis | |---------|-------------------|------------------|----------------| | **Age** | Usually >60 years | Any age, peak 60–80 | Any age | | **Onset** | Insidious → acute | Sudden, recurrent episodes | Acute, with fever | | **Palpable mass** | Common | Absent | Absent | | **CT finding: Shouldering** | Present (irregular margins) | Absent (smooth transition) | Absent | | **Fever** | Absent unless perforation | Absent | Usually present | | **Transition point** | Abrupt, irregular | Smooth, bird's-beak appearance | Gradual | **High-Yield:** "Shouldering" on CT — the abrupt transition from normal to narrowed bowel with irregular, asymmetric margins — is a hallmark of malignant obstruction and distinguishes it from benign causes. ### Pathophysiology of Malignant Obstruction 1. **Tumor growth** narrows the bowel lumen asymmetrically 2. **Desmoplastic reaction** (fibrous tissue response) creates irregular margins 3. **Proximal bowel dilation** occurs as contents accumulate 4. **Transition point** is typically at the tumor site, not distal to it ### Why Cancer Is Most Likely Here - **Age 72** — peak incidence of colorectal cancer - **Palpable mass** — suggests a large, infiltrative lesion - **Shouldering on CT** — pathognomonic for malignancy - **Afebrile** — rules out acute diverticulitis (which is usually febrile) - **No free air** — perforation has not yet occurred (yet) - **Rectosigmoid location** — most common site for colorectal cancer ### Clinical Pearl Colorectal cancers present with obstruction in 10–15% of cases, often in the left colon or rectosigmoid. Left-sided cancers are more likely to present with obstruction than right-sided lesions (which present with anemia and weight loss). The presence of a palpable mass increases suspicion significantly. ### Management Algorithm ```mermaid flowchart TD A[Suspected colorectal cancer obstruction]:::outcome --> B{Perforation or peritonitis?}:::decision B -->|Yes| C[Emergency surgery: resection + colostomy]:::action B -->|No| D{Right or left colon?}:::decision D -->|Right colon| E[Primary resection + anastomosis]:::action D -->|Left colon| F[Staged approach options]:::action F --> G[Option 1: Colostomy then delayed resection]:::action F --> H[Option 2: Self-expanding metal stent then elective resection]:::action F --> I[Option 3: Subtotal colectomy if high risk]:::action C --> J[Adjuvant chemotherapy + follow-up]:::action E --> J G --> J H --> J I --> J ``` **Mnemonic: MASS** — Malignancy, Age >60, Shouldering on imaging, Sudden obstruction in left colon.